U.S. patent application number 15/617315 was filed with the patent office on 2017-11-23 for magnetic nanoparticles, bulk nanocomposite magnets, and production thereof.
The applicant listed for this patent is BOARD OF REGENTS, THE UNIVERSITY OF TEXAS SYSTEM. Invention is credited to J. Ping LIU.
Application Number | 20170338015 15/617315 |
Document ID | / |
Family ID | 41725744 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170338015 |
Kind Code |
A1 |
LIU; J. Ping |
November 23, 2017 |
Magnetic Nanoparticles, Bulk Nanocomposite Magnets, and Production
Thereof
Abstract
Provided herein are systems, methods, and compositions for
magnetic nanoparticles and bulk nanocomposite magnets.
Inventors: |
LIU; J. Ping; (Arlington,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BOARD OF REGENTS, THE UNIVERSITY OF TEXAS SYSTEM |
Austin |
TX |
US |
|
|
Family ID: |
41725744 |
Appl. No.: |
15/617315 |
Filed: |
June 8, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14315677 |
Jun 26, 2014 |
9704625 |
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15617315 |
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12341656 |
Dec 22, 2008 |
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14315677 |
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61016353 |
Dec 21, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B82Y 30/00 20130101;
B29B 9/12 20130101; B22F 2009/043 20130101; B29B 2009/125 20130101;
H01F 1/0009 20130101; B82Y 25/00 20130101; H01F 10/123 20130101;
H01F 1/03 20130101; C22C 2202/02 20130101; H01F 41/00 20130101;
B22F 9/04 20130101; B22F 1/0025 20130101; H01F 41/0266 20130101;
B22F 2009/041 20130101; B22F 1/0018 20130101; H01F 1/0579
20130101 |
International
Class: |
H01F 1/03 20060101
H01F001/03; B22F 1/00 20060101 B22F001/00; H01F 10/12 20060101
H01F010/12; H01F 1/057 20060101 H01F001/057; B82Y 25/00 20110101
B82Y025/00; H01F 41/00 20060101 H01F041/00; B22F 9/04 20060101
B22F009/04; B29B 9/12 20060101 B29B009/12; B82Y 30/00 20110101
B82Y030/00; H01F 41/02 20060101 H01F041/02; H01F 1/00 20060101
H01F001/00 |
Goverment Interests
GOVERNMENT RIGHTS STATEMENT
[0002] This invention is made under Government support DoD/MURI
under Grant No. N00014-05-1-0497 and DoD/DARPA through ARO under
Grant No. DAAD-19-03-1-0038. The U.S. Government may have certain
rights to this invention.
Claims
1-9. (canceled)
10. A process of forming a plurality of magnetic nanoparticles
comprising: a. providing a powder with particles of a magnetic
material having a first size; b. dispersing the powder in a first
solvent with a surfactant; and c. ball milling the powder into a
plurality of nanoparticles of the magnetic material having a second
size.
11-21. (canceled)
22. The process of claim 10, wherein the first size is 1-45 .mu.m
and the second size is up to 50 nm.
23. The process of claim 10, wherein the plurality of nanoparticles
comprise elongated nanoparticles.
24. The process of claim 10, wherein the powder is selected from
one of FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, Co, Fe, Ni, CoFe,
NiFe, CoO, NiO, ferrites of formula MFe.sub.2O.sub.3 where M is Co
or Ni, FePt, CoPt, SmCo-based alloys including SmCo.sub.5,
Sm.sub.2Co.sub.17, Sm.sub.2Co.sub.7, and SmCo.sub.7, and rare
earth-FeB-based alloys of formula R.sub.2Fe.sub.14B where R is Nd
or Pr.
25. The process of claim 24, wherein the powder is a SmCo-based
alloy.
26. The process of claim 10, wherein the ball milling step includes
a weight ratio of ball to powder of 10:1 to 2:1.
27. The process of claim 10, wherein the amount of the surfactant
is 5-20% of the weight of the powder.
28. The process of claim 10, wherein the amount of the surfactant
is 8-10% of the weight of the powder.
29. The process of claim 10, wherein the surfactant comprises oleic
acid, oleyl amine, erucic acid, and/or linoleic acid.
30. The process of claim 10 further comprising: d. dispersing the
nanoparticles into a second solvent; and e. separating the
nanoparticles by a size selection process.
31. The process of claim 30, wherein the separating step comprises
spinning the nanoparticles dispersed in the second solvent to
obtain a first supernatant and a first slurry and separating the
first supernatant from the first slurry to obtain a plurality of
nanoparticles with an average size distribution of 4 to 10 nm
dispersed in the first supernatant.
32. The process of claim 31 further comprising a. washing the first
slurry with a solvent; b. dispersing the washed first slurry in a
solvent; c. statically settling down the dispersed solution of the
washed first slurry for 2 to 5 hours; d. spinning the settled down
solution of the first slurry to obtain a second supernatant and a
second slurry; and e. separating the second supernatant from the
second slurry to obtain a plurality of nanoparticles with an
average size of 10 to 15 nm.
33. The process of claim 31 further comprising: a. washing the
first slurry with a solvent; b. dispersing the washed first slurry
in a solvent; c. statically settling down the dispersed solution of
the washed first slurry for 20 to 30 minutes; d. spinning the
settled down solution of the first slurry to obtain a second
supernatant and a second slurry; and e. separating the second
supernatant from the second slurry to obtain a plurality of
nanoparticles with an average size greater than 20 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/315,677, filed Jun. 6, 2014, which is a
divisional of U.S. patent application Ser. No. 12/341,656, filed
Dec. 22, 2008, which claims priority to U.S. provisional
application Ser. No. 61/016,353, filed Dec. 21, 2007, each of which
is incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to magnetic
nanoparticles and more particularly to bulk magnetic
nanocomposites.
[0004] The size distribution of nanoparticles obtained by ball
milling can be wide compared with chemical synthesis methods.
Chemical synthesis methods have limited success in the synthesis of
hard magnetic nanoparticles of rare-earth compounds.
[0005] The grain size in nanocomposite magnets fabricated by
conventional top-down methods, including mechanical alloying and
rapid quenching, usually has a wide distribution, and can hardly be
controlled below the critical length. Fabrication of high density
bulks with controlled grain size and grain alignment of the hard
magnetic phases remains challenging. An alternative bottom-up
approach fabricates nanocomposite magnets with controllable
nanoscale morphology. The embodiments disclosed herein solves these
problems, as well as others.
SUMMARY OF THE INVENTION
[0006] Provided herein are systems, methods and compositions for
magnetic nanoparticles and bulk nanocomposite magnets.
[0007] The methods, systems, and compositions are set forth in part
in the description which follows, and in part will be obvious from
the description, or can be learned by practice of the methods,
compositions, and systems. The advantages of the methods,
compositions, and systems will be realized and attained by means of
the elements and combinations particularly pointed out in the
appended claims. It is to be understood that both the foregoing
general description and the following detailed description are
exemplary and explanatory only and are not restrictive of the
methods, compositions, and systems, as claimed.
BRIEF DESCRIPTION OF THE FIGURES
[0008] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate aspects of the
methods, compositions, and systems and together with the
description, serve to explain the principles of the methods,
compositions, and systems.
[0009] FIG. 1 is one embodiment of a bulk nanocomposite.
[0010] FIG. 2A is one embodiment of a warm compaction apparatus;
FIG. 2B is a perspective view of the first press system; FIG. 2C is
a schematic of the first press system; and FIG. 2D is an enlarged
schematic view of the central portion of the pressure vessel.
[0011] FIGS. 3A, 3B, 3C, 3D, 3E, and 3F are Transmission Electron
Microscope ("TEM") images of the as-synthesized FePt nanoparticles
of sizes 4 nm, 3 nm, 9 nm, 7 nm, 6 nm, and 2 nm, respectively.
[0012] FIGS. 4A and 4B are TEM images of the nanoparticles prepared
by surfactant-assisted ball milling of Fe powders for 1 hr and 5
hrs., respectively, and FIGS. 4C and 4D are TEM images of
nanoparticles prepared by surfactant-assisted ball milling
SmCo.sub.5-based powders for 5 hr. and 25 hrs., respectively.
[0013] FIGS. 5A-5D are TEM images of Sm.sub.2Co.sub.17
nanoparticles ground for 20 h in heptane with surfactants: FIG. 5A
the as-milled nanoparticles with sizes about 4-50 nm, FIG. 5B
nanoparticles separated by centrifugal separation, FIG. 5C
nanoparticles separated by 2-5 hrs. settling-down time; and FIG. 5D
nanoparticles separated after 20-30 mins settling-down time.
[0014] FIG. 6 is a graph showing the statistical size distributions
of the selected nanoparticles.
[0015] FIG. 7 is a graph of the X-ray scans of the ball-milled
Sm.sub.2Co.sub.17 samples.
[0016] FIG. 8 is a graph of the room-temperature magnetization
loops of the ball-milled Sm.sub.2Co.sub.17 nanoparticles with an
average size of 23, 13, and 6 nm; and the inset in FIG. 8 is a
graph of the particle size dependent coercivity of the nanoparticle
samples.
[0017] FIGS. 9A-9E are TEM images of salt-annealed FePt
nanoparticles of 9A: 2 nm; 9B: 4 nm; 9C: 6 nm; 9D: 8 nm; and 9E: 15
nm, where the Selected-Area Electron Diffraction ("SAED") patterns
are shown for the 2 and 15 nm salt-annealed particles as an inset
in FIGS. 9A and 9E respectively.
[0018] FIGS. 10A-10E are TEM images of bimagnetic
FePt/Fe.sub.3O.sub.4 core/shell nanoparticles where FIG. 10A is 4
nm FePt/1 nm Fe.sub.3O.sub.4 bimagnetic nanoparticles; FIG. 10B, is
6 nm FePt/2 nm Fe.sub.3O.sub.4 bimagnetic nanoparticles; FIG. 10C,
is 8 nm FePt/2 nm Fe.sub.3O.sub.4 bimagnetic nanoparticles; FIG.
10D, is 7 nm FePt/1 nm Fe.sub.3O.sub.4 bimagnetic nanoparticles;
and FIG. 10E, is 7 nm FePt/3 nm Fe.sub.3O.sub.4 bimagnetic
nanoparticles.
[0019] FIG. 11A is a TEM image of the FePt/FeCo core/shell
nanoparticle; and FIG. 11B is a graph of the hysteresis loops of
the FePt nanoparticle seeds and annealed FePt/FeCo
nanoparticle.
[0020] FIG. 12 is a graph of the dependence of density of the bulk
samples on compaction temperature T.sub.cp.
[0021] FIGS. 13A, 13B, and 13C are Scanning Electron Microscope
("SEM") images of the bulk samples compacted at 20, 400, and
600.degree. C., respectively.
[0022] FIG. 14 is a graph of the X-Ray Diffractometer ("XRD")
patterns of the bulk samples compacted at 20.degree. C. for (a),
400.degree. C. for (b), and 600.degree. C. for (c); where the
symbol (0) and full line represent the observed and calculated
x-ray diffraction profiles, respectively, and the vertical bars
represent the Bragg reflection positions of the observed phases
(from top to bottom: L1.sub.0, FePt, and Fe.sub.3O.sub.4, and where
the difference curve is plotted in the bottom.
[0023] FIG. 15 is a graph of the dependence S on T.sub.cp for the
bulk samples, where the dependence of S on annealing temperature of
the starting powder is also included for 10 min annealing.
[0024] FIG. 16 is a graph of the dependence of M.sub.s and H.sub.C
on T.sub.cp.
[0025] FIGS. 17A and 17B are graphs of the dependence of grain size
and microstrain on T.sub.cp for as-compacted samples and the grain
size of the annealed samples and the compacted L1.sub.0
nanoparticles.
[0026] FIGS. 18A-18D are TEM micrographs of bulk samples compacted
at different temperatures, 20.degree. C. in FIG. 18A, 400.degree.
C. in FIG. 18B, 600.degree. C. in FIG. 18C, and FIG. 18D is a graph
of SAED patterns at different temperatures.
[0027] FIG. 19A is a graph of .delta.m plots of the samples
compacted 400 and 600.degree. C. from the fcc nanoparticles, and
the inset of FIG. 19A is a graph of the respective hysteresis loops
of the samples compacted at 400 and 600.degree. C.; and FIG. 19B is
a graph of .delta.m plots of the separated L1.sub.0 particles and
samples compacted at 2.5 GPa, 200.degree. C. from the L1.sub.0
nanoparticles as well as the post-annealed bulk samples, where the
inset of the FIG. 19B is a graph of the hysteresis loops of the
L1.sub.0 nanoparticles.
[0028] FIGS. 20A, 20B, and 20C are graphs of the dependence of
M.sub.s, H.sub.C, and (BH).sub.max, respectively, on the annealing
temperature for 20, 400, and 600.degree. C. compacted samples.
[0029] FIG. 21 is a graph showing the comparison of the dependence
of density on the compaction temperature for (a) fcc FePt
nanoparticle under 2.5GPa; (b) fcc FePt nanoparticles under 3.8GPa;
(c) L1.sub.0 FePt nanoparticles; (d) MQ-15-7 ribbons; (e)
crystallized SmCo5/Fe nanocomposite powders; (f) amorphous SmCo/Fe
powders.
[0030] FIG. 22A is a graph of as-milled powders is SmCo amorphous
and very fine Fe nanocrystalline and FIG. 22B is a graph of
annealed powders is SmCo5+.alpha.-Fe nanocomposite.
[0031] FIGS. 23A, 23B, 23C, 23D, 23E, and 23F are SEM images of the
surface of the bulk SmCo/Fe samples compacted from the amorphous
powders at different temperatures, room temperature, 200.degree.
C., 300.degree. C., 400.degree. C., 500.degree. C., and 600.degree.
C., respectively.
[0032] FIG. 24 is a graph of the density of the SmCo/Fe samples
compared with the annealing temperature, where FIGS. 23A-23F
correspond to the amorphous red curve.
[0033] FIG. 25A is a graph of he dependence of density on
compaction temperature for amorphous SmCo--Fe powders, crystallized
SmCo--Fe powders, amorphous SmCo powders, and crystallized SmCo
powders; and FIG. 25B is a graph of the Vicker hardness ("HV")
dependence on compaction temperature for SmCo--Fe amorphous
powders, SmCo--Fe crystallized Fe powders, SmCo amorphous powders,
and SmCo crystallized powders.
[0034] FIGS. 26A, 26B, 26C, and 26D are SEM images of 20.degree.
C., 200.degree. C., 400.degree. C. and 600.degree. C.,
respectively, for compacted bulk nanocomposite SmCo.sub.5/Fe from
crystallized powders.
[0035] FIG. 27A is a graph of the dependence of grain size of
SmCo.sub.5 on the compaction temperature measured from XRD patterns
for SmCo--Fe amorphous powders, SmCo--Fe crystallized Fe powders,
SmCo amorphous powders, and SmCo crystallized powders; and FIG. 27B
is a graph of the dependence of grain size of and .alpha.-Fe grains
for SmCo--Fe amorphous powders and SmCo--Fe crystallized Fe
powders.
[0036] FIG. 28A is a graph of the XRD patterns for SmCo.sub.5+20%
Fe ball-milled for 4 hours and warm compacted at 550.degree. C. for
30 minutes; and FIG. 28B is a graph of the XRD patterns for
SmCo.sub.5+20% Fe ball-milled for 4 hours and warm compacted at
300.degree. C. for 30 minutes.
[0037] FIGS. 29A, 29B, and 29C are TEM images of 400.degree. C.,
600.degree. C. and 700.degree. C. compacted bulks from the
crystallized nanocomposite SmCo.sub.5/Fe powders, respectively.
[0038] FIG. 30A is an energy filter TEM image of the SmCo.sub.5/Fe
bulk nanocomposite; FIG. 30B is an energy filter TEM image showing
the element distribution of Co; FIG. 30C is a TEM image showing the
element distribution of Fe; and FIG. 30D is an TEM image shows the
element distribution of Sm.
[0039] FIG. 31A is a graph of the dependence of (BH).sub.max on
T.sub.cp of the bulk samples compacted from SmCo/Fe amorphous and
SmCo/Fe and SmCo crystallized powders; and FIG. 31B is a graph of
the second-quadrant B-H curves of the bulk nanocomposite and
single-phase magnets with maximum energy products of 16.5 MGOe for
SmCo.sub.5+20% Fe and the single phase counterpart includes 9.5
MGOe energy product.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] The methods, compositions, and systems can be understood
more readily by reference to the following detailed description of
the methods, compositions, and systems and the Examples included
therein and to the Figures and their previous and following
description.
[0041] The term "nanoparticles" includes particles having an
average size between about 2 and about 100 nm, preferably particles
having an average size between about 2 and about 100 nm. Most
preferably, the nanoparticles have an average size between about 2
and about 10 nm. The term "nanocomposites" includes composites
including more than one nanoparticle.
[0042] Generally speaking, a bulk nanocomposite magnet 10 comprises
at least one hard magnetic phase material 12 and at least one soft
magnetic phase material 14, as shown in FIG. 1. The hard phase
magnetic material 12 and the soft phase magnetic material 14 are
warm compacted with a warm compaction apparatus 20, as shown in
FIG. 2A, under pressure of at least about 0.1 to 6 GPa for a period
of time of at least about 1 minute to 30 hours at temperatures
ranging from about 20.degree. C. to 700.degree. C. to produce the
bulk nanocomposite 10. In one embodiment, the hard and soft phase
material may be modified in a pre-warm compaction step, i.e.,
before warm compacting, such as by mixing, heating, annealing, or
ball milling. The density of the compacted bulk magnetic materials
increases with the compaction pressure and temperature, while the
nanostructured morphology is retained. Phase transition
temperatures of the compacted materials can be altered in the
waiiii compaction technique and the phase transition facilitates
the consolidation of the bulk nanocomposite 10. Modest temperatures
maintain the chemical stability of the hard and soft phase
material. The warm compaction mechanism is based on surface/grain
boundary diffusion induced deformation, by optimizing Pressure,
Temperature, and Time (i.e. P-T-t combination), which is related to
the press design and the nanoparticle characteristics. In one
embodiment, the warm compaction and consolidation of the hard phase
and soft phase magnetic material includes magnetic nanoparticles
includes an average size of about 1 nm to about 100 nm. In another
embodiment, the hard and soft phase magnetic material includes
magnetic powder, where the magnetic powder includes magnetic
microparticles including an average size of about 1 .mu.m to about
100 .mu.m.
[0043] Interphase exchange coupling is enhanced upon the compaction
of the hard and soft phase material. Effective interphase magnetic
exchange coupling in the bulk nanocomposite is achieved by limiting
the dimensions of the soft phase material to a nanoscale critical
length, as determined by Zambano et al. "Dependence of Exchange
Coupling Interaction on Micromagnetic Constants in Hard/soft
Magnetic Bilayer Systems", Physical Review B, 75: 144429-1-7
(2007), herein incorporated by reference. In one embodiment,
post-annealing the bulk nanocomposite 10 under a forming gas at an
elevated temperature to improve the magnetic performance of the
bulk nanocomposite 10 owing to interface modification.
Additionally, grain growth and grain size is controlled with warm
compaction by selecting P-T-t profiles to ensure chemical stability
of the magnetic nanoparticles. The bulk nanocomposite includes a
controlled grain size for intergrain magnetic exchange coupling and
high energy products.
[0044] "Bulk" is a term used in the application to mean any
nanocomposite magnet having a dimension of at least about
0.5.times.0.5.times.0.5 mm, preferably at least about
1.times.1.times.1 mm, most preferably at least about
3.times.1.2.times.0.5 mm, alternatively between
6.times.1.5.times.0.5 mm. "Bulk nanocomposite" is also referred to
as a "compact". The bulk nanocomposite 10 may include a general
three dimensional shape including a rectangular shape, cubical,
cuboidal, cylindrical, polyhedronal, and the like. The "soft phase
material" 14 comprises at least one of FeO, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, Co, Fe, Ni, CoFe, NiFe, CoO, NiO and other related
oxides including doped ferrites including, but not limited to
MFe.sub.2O.sub.3 where M comprises one of Co and Ni. The "hard
phase material" 12 comprises at least one of FePt, CoPt, SmCo-based
alloys, including SmCo.sub.5, Sm.sub.2Co.sub.17, Sm.sub.2Co.sub.7,
and SmCo.sub.7, and rare earth-FeB-based alloys, including
R.sub.2Fe.sub.14B, where R.dbd.Nd or Pr, i.e. Nd--FeB and Pr--FeB.
The hard phase material and the soft phase material may be mixed
pre-warm compaction at a mass ratio of 10:1 to about 5:1,
alternatively about 8:1 to about 2:1, alternatively about 1:1. The
mass ratio used for mixing the hard and soft phase material depends
on the hard and soft phase material selected for warm
compaction.
[0045] For nanoparticles, the limit for density (".rho.p") of a
randomly packed nanoparticle system is only 64% if no deformation
is involved. To obtain a higher .rho.p, plastic deformation of the
particles is necessary and is obtained in the warm compaction
process. For nanoparticles, the deformation is not as easy as for
large nanoparticles because of the reduced dislocations in the
nanoparticles, and a lower density values obtained in large
nanoparticles warm compacted into a bulk nanocomposite. The bulk
samples .rho.p is dependent on the compaction temperature ("Tcp")
under different pressures. The density of the bulk composite
increases monotonously with compaction temperature.
[0046] In one embodiment, the bulk nanocomposite magnets are
prepared by warm compacting hard phase and soft phase magnetic
nanoparticles and/or nanocomposite particles. Generally speaking,
the warm compaction apparatus 20 is shown in FIG. 2A. The warm
compaction apparatus 20 includes a first press system 28, a
hydraulic pump 30, a second press system 40, and a temperature
controller 50. The first press system 28 is operably coupled to the
temperature controller 50 to control the temperature of the first
press system 28. The second press system 40 is operably coupled to
the hydraulic pump 30 to provide additional pressure to the first
press system 28. The first press system 28 is shown in FIG. 2B, and
includes a pressure generator 22 with at least one piston, a
pressure plate 26 with a plurality of heating elements, a pressing
chamber 24, and a temperature controller (not shown). FIG. 2C shows
the schematic portion of the pressure chamber 24, which includes an
end load ram 60, a main hydraulic ram 62, an end load spacer 64, a
thermocouple spacer 66, a stack top plate 68, a pressure vessel 70,
and a bridge 72. FIG. 2D shows an enlarged view of the central
portion of the pressure vessel 70, which includes a base plug 74, a
pyrophyllite sleeve 76, a halite 78, a pyrex sleeve 80, a graphite
furnace 82, a powder portion 84, a sample cup and lid 86, and a
thermocouple well 88. The powder portion 84 may include alumina or
MgO powder. The sample cup and lid 86 is where the nanocomposites
are placed for warm compaction. The bulk nanocomposite magnet may
also be modified post-warm compaction to enhance magnetic
characteristics or microstructure.
[0047] The hard and soft phase material may comprise magnetic
nanoparticles. The magnetic nanoparticles may include varied
characteristics to enhance the magnetic properties of the bulk
nanocomposite 10. For example, the magnetic nanoparticle
microstructure or morphology may include, but is not limited to, a
specific size, such as spherical, aspherical, elongated nanorods,
bricklike, wire, cube, hexagonal and tetragonal structures,
plate-like structures, monodisperse, polycrystalline,
monocrystalline, a specific grain size, and shell of a
non-magnetic, antiferromagnetic, or ferro/ferri-magnetic shell,
which is otherwise known as a core/shell. The magnetic
characteristics may include, but are not limited to, a specific
coercivity, magnetocrystalline anisotropy, unsaturated loops,
superparamagnetic, ferromagnetic, low or high remanence ratio,
single phase-like magnetization, amorphous structure, and exchange
coupling. In one embodiment, the hard phase material may comprise a
plurality of FePt nanoparticles with fcc structure, fct structure,
or L1.sub.0-phase structure. In another embodiment, the hard and
soft phase material may comprise amorphous or crystallized
powders.
[0048] The magnetic nanoparticles may be modified pre-warm
compaction to obtain the magnetic characteristics or
microstructures, such as by a heating, annealing, or a ball milling
step. In one pre-warm compaction step, the ball milling step may
include a gas or a liquid as the media for ball milling for the
microstructure or morphology. In one embodiment, the pre-warm
compaction step includes synthesizing the particles (nano or micro)
and mixing the hard and soft phase particles by ball milling (with
a gas or liquid as the media) to foim the required nanocomposite
morphology. Magnetic nanoparticles may be synthesized or modified
by any known method to obtain magnetic characteristics or
microstructure, none which are intended to limit the scope of the
bulk nanocomposite magnets prepared by warm compaction, some of
which are described below.
[0049] Magnetic Nanoparticles
[0050] In one embodiment, the magnetic nanoparticles are
synthesized by an airless chemical solution procedure otherwise
known as chemical reduction/thermal decomposition. The hard phase
and soft phase magnetic nanoparticle material may be synthesized by
this method. One example of the standard airless technique uses an
argon atmosphere and 0.5 mmol of platinum acetylacetonate
("Pt(acac).sub.2") is added to 125 mL flask containing a magnetic
stir bar and mixed with 20 mL of octyl/benzyl ether, as disclosed
in Nandwana et al. "Size and Shape Control of Monodisperse FePt
Nanoparticles" J. Phys. Chem. C, 111: 4185-4189 (2007), herein
incorporated by reference. After purging with argon for 30 min at
room temperature, the flask is heated up to 120.degree. C. for 10
min and a designated amount of oleic acid and oleylamine is added.
Iron pentacarbonyl ("Fe(CO).sub.5") or iron acetylacetonate
("Fe(acac).sub.3") is used as an iron precursor. Iron
acetylacetonate (0.5 mmol) is added at room temperature while iron
pentacarbonyl (1.0 mmol) is added at 120.degree. C. when the
platinum precursor dissolved completely. The dissolution of
Pt(acac).sub.2 in solvent could be followed experimentally by the
change of color of the solution from off yellow to transparent
yellow. After the addition of Fe(CO).sub.5, the color transition
from golden to black suggested formation of nanoparticles in the
solution. Then it is heated to 298.degree. C. for 1 h before
cooling to room temperature under the argon blanket. Argon gas is
flowed throughout the experiment. The heating rate is varied from 1
to 15.degree. C. per minute according to the experimental
design.
[0051] The black product is precipitated by adding ethanol and
separated by centrifugation and redispersed in hexane. To achieve
the highest purity, extra ethanol is added in this dispersion and
the dispersion is centrifuged again. Because all the particles are
quite homogeneous, size selection is not necessary. After washing
the particles in ethanol three or more times, they are dispersed in
hexane and stored in glass bottles under refrigeration. Samples for
magnetic characterization are prepared by depositing a drop of the
final hexane dispersion on a 3.times.3 mm silicon substrate,
evaporating the solvent at room temperature and further drying in
vacuum, which led to the formation of FePt nanoparticle-assembled
thin films. The samples are then annealed at 650.degree. C. for 1 h
under the flow of forming gas (Ar+7% H.sub.2) in a tube
furnace.
[0052] Fine-tuning of the sizes of the FePt nanoparticles between 2
and 9 nm may be achieved by controlling the surfactant to metal
ratio and the heating rate, as disclosed in Nandwana et al. "Size
and Shape Control of Monodisperse FePt Nanoparticles" J. Phys.
Chem. C, 111: 4185-4189 (2007), herein incorporated by reference.
As shown in FIG. 3A, when the heating rate of the reaction is
maintained at 5.degree. C., FePt nanoparticles with average size of
4 nm are obtained when the molar ratio of surfactant to
Pt(acac).sub.2 is 1. By decreasing the molar ratio to 0.75, the
nanoparticle size decreases to 3 nm, as shown in FIG. 3B. By
increasing the molar ratio to 10, the nanoparticle size increased
to 9 nm, as shown in FIG. 3C. And as the heating rate increased
from 5 to 10 and then to 15.degree. C./min, the average
nanoparticle size decreases from 8 to 7 nm, as shown in FIG. 3D,
and then to 6 nm when using the surfactant ratio of 5, as shown in
FIG. 3E. The increased heating rate increases the nucleation rate,
i.e. more nuclei forms at the initial stage. Alternatively, when
the heating rate is decreased to 1.degree. C./min, the nanoparticle
size is decreased to 5 nm. And when Fe(acac).sub.3 is substituted
for Fe(CO).sub.5 as a precursor, nanoparticles of 2 nm are
obtained, as shown in FIG. 3F. Alternatively, to make larger FePt
particles, a seed-mediated growth method is used. This is performed
by first making monodisperse 3-4 nm seed FePt particles, and then
adding more Fe and Pt precursors to enlarge the existing FePt
nanoparticle seeds to obtain the desired sizes. Alternatively,
control of the size of the FePt nanoparticles is obtained through
one step simultaneous thermal decomposition of FeCO.sub.5 and
reduction of Pt(acac).sub.2 in the absence of 1, 2-alkanedial.
1,2-alkanediol leads to the facile reduction of Pt(acac).sub.2 to
Pt, resulting in fast nucleation of FePt and consumption of metal
precursors resulting in smaller particles. Therefore, removal of
the additional reducing agent slows down the nucleation rate,
allowing more metal precursors to deposit around the nuclei, and
leading to a larger nanoparticle size. The nanoparticles can be
annealed in forming gas of Ar+7% H.sub.2 at 650.degree. C. for 1
hr., which increases coercivity based on the fcc-fct phase
transition. The disordered face-centered cubic ("fcc") structure
has low magnetocrystalline anisotropy, so heat treatment is
necessary to convert the fcc structure to the ordered face-centered
tetragonal ("fct") structure, described by the chemical-ordering
parameter S. The L1.sub.0-FePt phase is based on the crystalline
ordering of the fct structure and the L1.sub.0-FePt phase has high
uniaxial magnetocrystalline anisotropy.
[0053] Samples for magnetic measurements are prepared by embedding
the nanoparticles in epoxy inside the glove box. Magnetic
measurements at room temperature are performed using an alternating
gradient magnetometer with measuring field up to 14 kOe, and at 5 K
using a superconducting quantum interference device ("SQUID") with
measuring field up to 70 kOe. Structural and morphological
characterizations are performed using transmission electron
microscope ("TEM"), and x-ray diffractometer ("XRD"). Compositional
characterizations are performed using energy dispersive x-ray
spectroscopy ("EDX") and inductively coupled plasma ("ICP").
[0054] In another embodiment, the magnetic nanoparticles are
produced by a polyol process. The use of a diol or polyalcohol (for
example, ethylene glycol) to reduce metal salts to metal particles
is also referred to as the "polyol process". By mixing and heating
both an iron salt and a platinum salt with the polyol, high-quality
FePt nanoparticles can be produced. Alternatively, monodisperse
SmCo.sub.5 nanoparticles are synthesized by coupling the polyol
reduction of samarium acetylatacetonate, ("Sm(acac).sub.3"), with
the thermal decomposition of Co.sub.2(CO).sub.8. For FePt, a slight
modification of the decomposition/reduction condition by replacing
Fe(CO).sub.5 with Fe(acac).sub.2 or Fe(acac).sub.3 leads to
monodisperse 2-3 nm diameter FePt nanoparticles. The stronger
organic reducing agent hydrazine (N.sub.2H.sub.4) may be used to
reduce metal salts and form FePt nanoparticle in water at low
temperature. In this synthesis, H.sub.2 PtCl.sub.6.H.sub.2O and
FeCl.sub.2.H.sub.2O, together with hydrazine and a surfactant, such
as sodium dodecyl sulfate ("SDS") or cetyltrimethylammonium bromide
("CTAB"), are mixed in water. The hydrazine reduces the metal
cations at 70.degree. C., resulting in fcc-structured FePt
nanoparticles. One example of a modified polyol process is
disclosed in Elkins et al "A Novel Approach to Synthesis of FePt
Magnetic Nanoparticles" Journal of Nano Research, 1: 23-29 (2008),
herein incorporated by reference. The modified polyol process
includes Pt(acac).sub.2 and Fe(acac).sub.3 in the molar ratio 1:1
is taken in a 125 mL flask containing a PTFE coated magnetic stir
bar at room temperature. 1, 2-hexadecanediol (5 times mole amount
of Pt(acac).sub.2 and Fe(acac).sub.3) is added to the flask. 30 mL
of dioctyl ether is then transferred into the flask and the
contents are stirred while purging with Ar for 30 mins. at room
temperature. The flask is then heated to 200.degree. C. at
6.degree. C./min. by use of a Glas-Col hemispherical heating mantel
connected to a programmable heat controller using a type J
thermocouple. Once the temperature reached 200.degree. C., the
flask is kept at this temperature for 30 minutes. After the 30
minute hold, the flask is heated to 295.degree. C. at a rate of
approximately 5.degree. C. per minute. The flask is maintained at a
refluxing temperature of 295.degree. C. for 30 min. before cooling
down to room temperature under the Ar purge. Afterwards, all
handling is performed open to the atmosphere. Purification of the
nanoparticles is accomplished as follows: 5 mL of the dispersion
taken from the flask is added to 20-25 mL of ethyl alcohol ("EtOH")
and the mixture is centrifuged 6000 rpm for 15 min. The supernatant
is discarded and the precipitate is redispersed in 10 mL of hexane.
The dark brown dispersion is stored under refrigeration at
approximately 10.degree. C. The FePt nanoparticles include an
average size of 2 nm and have a spherical shape and narrow size
distribution. The XRD patterns show that the FePt nanoparticles
have a chemically disordered fcc structure. From the peak width of
the XRD pattern, the average nanoparticle diameter of 1.6 nm is
calculated using the Scherer foimula.
[0055] In another embodiment, the magnetic nanoparticles are
synthesized by surfactant-assisted ball milling, as disclosed in
Chakka et al. "Magnetic Nanoparticles Produced by
Surfactant-Assisted Ball Milling, J. Applied Physics, 99: 08E912
(2006), herein incorporated by reference. The starting powders have
nanoparticle sizes from .sup..about.10 to 45 .mu.m (-325 mesh). Fe
powders with 98% purity and Co powders with 99.5% purity may be
used. Alloy powders of Sm--Co (1:5 and 2:17), NdFeB (2:14:1), and
FeCo are prepared by arc melting followed by grinding. The milling
process and handling of the starting powders and the milled
particles are carried out in an oxygen-free inert environment. In
one embodiment, argon gas is used inside a glove box; alternatively
other inert gases may be used included N2 and the like. The
starting powders are milled under a liquid or organic solvent,
including ethanol or heptane. Heptane may include purity
approximately 90-100%, alternatively 99.8%. The starting powders
are also ball milled with a surfactant. Surfactants are
characterized by having one long hydrocarbon chain per surfactant
headgroup. In one embodiment, oleic acid and oleyl amine are used
as surfactants during milling, where oleic acid may include a
purity of 85-100%, alternatively 90% and oleyl amine may include a
purity of 90-100%, alternatively greater than 98%. Other
surfactants that may be used include, but are not limited,
derivatives of oleic acid, erucic acid, linoleic acid, and the
other long chain carboxylic acids surfactants. The surfactants used
are absorbed by the fresh surface of particles crushed during the
ball milling, leading to a surface modification for the ground
particles. The amount of surfactant used is approximately 5-20% by
weight of the starting powder, alternatively 7-15% or 10-12% by
weight of the starting powder may be used. The mixtures were ball
milled in a vibrating or rolling vial containing balls and the
balls may include stainless steel, hardened steel, carbide or other
ceramic balls. In one embodiment, high energy Spex 8000M mill with
the milling vial and the balls made of 440 C hardened steel are
used for milling the nanoparticles. The milling durations are from
1 to 50 hrs. The milling time and vibrating strength are adjusted
to form the desired nanocomposite morphology. The ball to powder
weight ratio may include 10:1 to about 2:1, alternatively 6:0.5 to
about 7:3, or 5:1.
[0056] When the surfactant is used along with heptane during
milling a colored liquid is obtained along with coarse particles,
referred to as a slurry, which remain as sediment at the bottom of
the milling vial after milling. The colored liquid contains a
dispersion of nanoparticles smaller than 30 nm. As shown in FIG.
4A, TEM images of the nanoparticles prepared by milling Fe powders
for different milling durations 1, 5, 15, 25, and 50 hr. using
oleic acid as the surfactant show spherical or aspherical
nanoparticles in sizes ranging from 3 to 9 nm. There is no
significant change in morphology of the nanoparticles with increase
in milling time up to 50 h for the Fe powders. FIG. 4B shows
nanoparticles with very a narrow size distribution 4-6 nm obtained
by milling Fe powders for 1 h and by using stainless steel 316 SS
balls instead of hardened steel balls. As seen in FIG. 4B, the
nanoparticles self assemble into a regular pattern, which can be
achieved by controlled evaporation of the solvent after depositing
the nanoparticle dispersion liquid on the TEM grid. Similarly, FeCo
powders show ultrafine spherical nanoparticles with a narrow
nanoparticle size distribution ranging from 1.7 to 4.0 nm for
samples ball milled using oleic acid. However, milling of Co
particles with oleic acid results in a jelly-like mass, not a
liquid with nanoparticle dispersion, which makes separating the
nanoparticles from the solution difficult. A liquid with
nanoparticle dispersion is obtained by milling Co particles with
oleyl amine and the morphology of the nanoparticles shows mostly
aspherical nanoparticles with a few elongated nanorods for the 5
hr.-milled sample. With increasing milling time, the percentage of
elongated nanorods increased. For the sample prepared by milling
for 10 hr., the aspherical particles with size ranging from 2 to 8
nm are obtained. Elongated rods with width of 2-3 nm and lengths
from 10 to 18 nm aspect ratios 5-9 are also obtained. Such
elongated nanostructures have not been observed in the case of
nanoparticles prepared by milling Fe and FeCo powders.
Superparamagnetic behavior is observed at room temperature for the
ball milled Fe, Co, and FeCo nanoparticles. Magnetic measurements
at 5 K using SQUID show some hysteresis with coercivities up to 400
Oe, and an unsaturated loop even at 70 kOe, implying a combination
of superparamagnetic and ferromagnetic behavior at 5 K.
[0057] As shown in FIGS. 4C and 4D, the TEM images of the
nanoparticles prepared by ball milling SmCo.sub.5-based powders
show that the morphology of the nanoparticles varies with increased
milling time. Nanoparticles with size ranging from 3 to 13 nm are
included in the 5 h-milled sample, which are mostly irregular and
aspherical with a very small percentage of elongated particles.
Elongated rod-shaped nanoparticles are included when the milling
time is increased to 15 hr., along with other irregular particles.
The percentage of elongated rod-shaped nanoparticles is increased
further when the milling time is 25 hr. Elongated rod-shaped
nanoparticles with length from 7 to 20 nm and width from 3 to 6 nm
are included when the milling time is 25 h and the aspect ratios of
the nanorods varies from 1.5 to 5. As shown in FIG. 4D, the
rod-shaped nanoparticles tend to align parallel to each other on
the TEM grid. Similar nanorod formations are also found in the case
of nanoparticles prepared by milling Sm.sub.2Co.sub.17 and
Nd.sub.2Fe.sub.14B-based powders with oleic acid. The nanorods are
produced by fracture along some preferred crystalline orientation
or anisotropic growth of the nanoparticles during the milling and
increased temperatures locally inside the milling vial facilitate
the anisotropic growth. Materials with hexagonal structure,
SmCo.sub.5, Sm.sub.2Co.sub.17, Co, and tetragonal structures,
Nd.sub.2Fe.sub.14B, have a preferred orientation for fracture that
are the close-packed planes [(0001) for hcp], and form plate-like
structures, which upon further milling results in the formation of
elongated nanoparticles. Magnetic properties of the ball milled
SmCo.sub.5, Sm.sub.2Co.sub.17 nanoparticles show hysteresis at room
temperature with low coercivities (>100 Oe) and very low
remanence ratio (remanence ratio=M.sub.r/M.sub.s) of less than 0.1.
At 5 K the coercivity and the remanence ratio increased to
H.sub.c=1.6 kOe, M.sub.r/M.sub.s=0.42 for SmCo.sub.5-based 25
h-milled samples. The remanence ratio increases with milling time.
The magnetization curves of the nanoparticles by milling
SmCo.sub.5-based powder measured at 5 K show that the loops that
M.sub.r/M.sub.s values increase with milling time, from
M.sub.r/M.sub.s=0.08 for a 5 hr.-milled sample to
M.sub.r/M.sub.s=0.42 for the 25 hr.-milled sample. The NdFeB
nanoparticles also include the remanence ratio enhancement with
milling time at low temperatures. Remanence enhancement is due to
the increasing percentage of elongated rod shapes and higher aspect
ratios for longer milling time and/or the formation of most of the
nanoparticles from the crystalline regions, which are the core of
the primary nanoparticle, rather than the disordered boundary
regions with increasing milling time.
[0058] In another embodiment, surfactant assisted ball milling
includes starting powders of SmCo.sub.5 and Sm.sub.2Co.sub.17 with
the starting powder sizes from approximately 1-45 .mu.m, as
disclosed in Wang et al. "Sm--Co hard magnetic nanoparticles
prepared by surfactant-assisted ball milling" Nanotechnology 18:
465701 (2007), herein incorporated by reference. Organic solvent of
heptane (.sup..about.99.8% purity) is used as the milling media and
oleic acid (.sup.1890%) and oleyl-amine (.sup..about.98%) are used
as the surfactants during milling. The powders are ground in a
milling vial with balls made of 440 C hardened steel by using a
Spex 8000M high energy ball milling machine. Milling process and
handling of the starting materials and the milled products are
carried out in an argon gas environment inside a glove box to
protect the particles from oxidation. Typical milling duration used
is 20 hrs. with balls of 1/4 inch in diameter. The weight ratio of
powder to ball is set as 1:10. The amount of surfactant used is
approximately 8%-10% and the used solvent is about 55% of the
weight of the starting powder, respectively. The ground slurry is
then dispersed into heptane solvent by ultrasonic vibration and
transferred to a 50 ml centrifugal tube for size selection. Fe, Co,
FeCo, and Nd.sub.2Fe.sub.14B nanoparticles may also be used in the
size selection process.
[0059] As shown in FIG. 5A, a TEM image of the Sm2CO.sub.17 sample
ball milled for 20 hrs. show irregular shapes and a wide size
distribution from several nanometers to larger than 50 nm.
Different sizes of the nanoparticles are separated using
centrifugal separation and controlling the "settling down" time of
the nanoparticle solutions. "Settling down" time is the time the
nanoparticles settle down after being dispersed by ultrasonic
vibration. Small size nanoparticles float in the heptane solvent
after centrifugation of 3000 rotations per minute ("rpm") for 25
min., which is relatively 1600 g in centrifugal force. Then the
small size nanoparticles are separated by taking the supernatant
after the centrifugation procedure 3000 rpm for 25 min. As shown in
FIG. 5B, the morphology of the small size nanoparticles are shown
with an average diameter of 6 nm. Large size nanoparticles are
obtained by removing the small size nanoparticles in the
supernatant and washing the slurry one time with heptane to remove
any of the remaining small size nanoparticles. The washed slurry is
then transferred to a surfactant-coated centrifugal tube and
dispersed with heptane by ultrasonic vibration. The dispersed
solution is then statically settled down for approximately 2-5
hrs., i.e. a settling down time of 2-5 hrs. After the settling down
time of 2-5 hrs., a low speed centrifugal separation of 250-500 rpm
(.sup..about.45 g of centrifugal force) removes the largest
particles. The morphology of the nanoparticles remained after the
settling-down process is shown in FIG. 5C, where nanoparticles with
average size of 13 nm are obtained. Larger size nanoparticles can
be obtained with an average diameter of 23 nm with a shorter
settling-down time of 20-30 min. As shown in FIG. 5D, the larger
size nanoparticles display the typical morphology with an average
diameter of size 23 nm.
[0060] As shown in FIG. 6, a graph of the statistical size
distributions of the nanoparticles after the size selection
process. The charts 1, 2 and 3 correspond to the nanoparticles
shown in FIGS. 5B, 5C, and 5D, respectively. Compared with the
as-milled particles, the nanoparticles after the size selection
have much more narrow size distributions. In FIG. 7, X-ray
diffraction patterns of Sm2Co.sub.17 nanoparticles of different
sizes (6 nm, 13 nm, and 23 nm) obtained by the size-selection
process are shown and the pattern of the starting Sm.sub.2Co.sub.17
powder is shown for comparison. There are no peaks from oxides and
pure iron and cobalt are presented in the diffraction patterns,
indicating that the prepared nanoparticles are effectively
protected from oxidation by handling the solution in the glove box
and embedding the particles in epoxy. SiO.sub.2 coating improves
the oxidation resistance of the ground Sm--Co nanoparticles in air.
The diffraction patterns show that no detectable contamination or
decomposition in the particles has occurred. The EDX measurements
show that the compositions of the nanoparticle samples are close to
the compositions of the starting Sm.sub.2Co.sub.17 powder. FIG. 7
also shows that the diffraction peaks are broadened with the
decreasing nanoparticle size.
[0061] In FIG. 8, the magnetization loops of the Sm.sub.2Co.sub.17
nanoparticles with different sizes, 6 nm, 13 nm, and 23 nm are
shown by the blue, red, and black curves respectively. All the
loops show a single-phase-like magnetization behavior with no kinks
on the demagnetization curves, indicating no second phases in the
particles. The inset in FIG. 8 shows the size-dependent coercivity
of the particles. The starting powder, 23 nm, 13 nm and 6 nm
samples have coercivity of 20 kOe, 3.1 kOe, 2.4 kOe and 170 Oe at
room temperature, respectively. These values are significantly
higher compared to the reported values of Sm--Co nanoparticles
synthesized by other methods. The size dependent coercivity in
these nanoparticles may be due to the increased defects in the
smaller particles lower magnetocrystalline anisotropy. In addition,
local strains can cause low-energy nucleation sites. Additionally,
the ball milling may lead to partial amorphorization in the
nanoparticles and the smaller particles have more amorphous
structure which leads to reduced coercivity.
[0062] In another embodiment, the nanoparticles can be synthesized
by a salt-matrix annealing technique, as disclosed in Rong et. al.
"Size-dependent chemical and magnetic ordering in L1.sub.0 FePt
nanoparticles" Advanced Materials, 18: 2984-2988 (2006), herein
incorporated by reference. The fcc-structured FePt nanoparticles
with different sizes (from 2 to 15 nm) are synthesized using the
airless chemical-solution method with adjusted synthetic
parameters. The fcc particles are then mixed with ball-milled NaCl
powder in hexane or another organic solvent with the assistance of
surfactants. Dry mixtures of FePt particles and NaCl powders are
obtained after the solvent is evaporated completely. The mixtures
are then annealed in a forming gas (93% Ar+7% H.sub.2) at different
temperatures for different times. After the annealing, the NaCl
powder is washed away by deionized water and the FePt nanoparticles
are recovered and dispersed in organic solvents, such as
cyclohexane or ethanol, in the presence of surfactants. The
elemental composition analyses, using inductively coupled
plasma-optical emission spectroscopy ("ICP-OES"), show that there
is negligible NaCl contamination in the salt-matrix-annealed FePt
nanoparticles and the nanoparticle composition is
Fe.sub.52Pt.sub.48. A transmission electron microscope is used to
analyze the morphology and crystalline structures. X-ray
diffraction ("XRD") is used to determine the phase transition, the
long-range ordering parameters, the grain size, and the
nanoparticle size. The magnetic hysteresis loops are measured with
a magnetic properties measurement system ("MPMS") from specimens of
a mixture of epoxy and the magnetic nanoparticles. Curie
temperatures are measured by a physical properties measurement
system ("PPMS") with high-temperature and high-vacuum vibrating
sample magnetometer. With this technique, nanoparticle aggregation
during the phase transfoimation has been avoided so that the true
size-dependent properties of the fct phase can be measured. FePt
nanoparticles with different sizes are annealed in a salt matrix at
973 K for 4 hr. Nanoparticles with nominal diameters of 2, 4, 6, 8,
and 15 nm, can be obtained, and TEM images are shown in FIGS. 9A,
9B, 9C, 9D, and 9E for 2, 4, 6, 8, and 15 nm respectively. Annealed
nanoparticles are monodisperse with a standard deviation of 5-10%
in diameter. TEM observations also revealed that when the
nanoparticle size is smaller than or equal to 8 nm, the fct
nanoparticles are monocrystalline, whereas the 15 nm fct particles
are polycrystalline.
[0063] As shown in FIGS. 9B-9E, L1.sub.0 nanoparticles are
dispersed very well without agglomeration despite the dipolar
interaction between the particles, if a solvent with high viscosity
is chosen and if the solution is diluted. Extensive TEM and X-ray
diffraction ("XRD") analyses shows salt-matrix annealing can be
applied to heat-treatments of the FePt nanoparticles without
leading to nanoparticle agglomeration and sintering, if a suitable
salt-to-particle ratio and proper annealing conditions are chosen,
as disclosed in Narayan et al., "Hard Magnetic FePt Nanoparticles
by Salt-Matrix Annealing", J. Applied Physics, 99: 08E911 (2006),
herein incorporated by reference. XRD patterns of the 4 nm,
as-synthesized, fcc-structured nanoparticles and the particles
annealed in a salt matrix at 873 K for 2 hr., 973 K for 2 hr., and
973 K for 4 hr. show the positions of the (111) peaks shift in the
higher-angle direction with increasing annealing temperature and
time. This shift is caused by the change of lattice parameters
during the phase transition from fcc to fct. The (001) and (110)
peaks are the characteristic superlattice peaks of the ordered fct
phase and developed with increasing annealing temperature and time.
For samples annealed at 973 K for 4 hr., the superlattice peaks
(001) and (110) are fully developed, implying an fcc-fct
transition. XRD patterns of the converted fct phase of 6, 8, and 15
nm nanoparticles annealed at 973 K for 4 hr. in a salt matrix. The
grain sizes estimated by the Scherrer formula are about 4.7, 6.8,
8.2, and 13.3 nm for the 4, 6, 8, and 15 nm nanoparticles, Selected
area electron diffraction ("SAED") is used to identify the
crystalline structure of the particles, especially for the smaller
particles. The SAED patterns show that the annealed nanoparticles
are L1.sub.0-fct phase if the nanoparticle size is equal to or
larger than 4 nm, as shown in the inset of FIG. 9E, by the (001)
and (110) rings. The SAED pattern of 2 nm particles as shown in the
inset of FIG. 9A, does not show (001) and (110) rings.
[0064] Additionally, the magnetic nanoparticles may be coated with
a shell of a non-magnetic, antiferromagnetic, or
ferro/ferri-magnetic shell, which is otherwise known as a
core/shell nanoparticle. A nonmagnetic coating is used routinely
for magnetic core stabilization and surface functionalization for
biomedical applications. An antiferromagnetic coating over a
ferromagnetic core leads to exchange bias (a shift of the
hysteresis loop along the field axis), and improvements in the
thermal stability of the core. Compared with these two different
types of core/shell systems, a bimagnetic core/shell one, where
both core and shell are strongly magnetic (ferro- or
ferri-magnetic) can be applied in electromagnetic and permanent
magnetic applications. The intimate contact between the core and
shell leads to effective exchange coupling and therefore
cooperative magnetic switching, facilitating the fabrication of
nanostructured magnetic materials with tunable properties. The
bimagnetic core/shell also improves the compressibility in the warm
compaction technique by controlling the soft phase shell thickness
and composition. The ductility of the core/shell nanoparticles is
increased compared with single-hard-phase nanoparticles. The
increased ductility increases the warm compaction of core/shell
nanoparticles. The core/shell nanoparticles can be also deformed to
form texture during the warm compaction process.
[0065] In one example of coating the magnetic nanoparticles,
monodisperse FePt nanoparticles are synthesized by thermal
decomposition of Fe(CO).sub.5 and polyol reduction of
Pt(acaca).sub.3, as disclosed in Chaubey et al., "Synthesis and
Characterization of Bimagnetic Bricklike Nanoparticles" Chem.
Mater. 20; 475-478, (2008), herein incorporated by reference. An
iron oxide coating is achieved via mixing and heating the FePt
nanoparticle seeds with Fe(acac).sub.3/polyol, or
Co(acac).sub.2/Fe(acac).sub.3/polyol precursors in phenyl ether in
the presence of 1,2-hexadecanediol, oleic acid, and oleylamine.
FePt/Fe.sub.3O.sub.4 nanoparticles are obtained by refluxing the
FePt nanoparticle seeds with Fe(acac).sub.3/polyol in the reaction
mixture at 265.degree. C. for 30 min. Bimagnetic
FePt/Fe.sub.3O.sub.4 nanoparticles are isolated by centrifugation.
The size of the soft phase could be controlled by varying the
material ratio of FePt nanoparticle seeds to Fe(acac).sub.3/polyol
metal precursors. Reductive annealing can transform the bimagnetic
nanoparticles into a hard magnetic nanocomposite with an enhanced
energy product. In FIG. 10A, 4 nm FePt/1 nm Fe.sub.3O.sub.4
bimagnetic nanoparticles are shown by TEM. In FIG. 10B, 6 nm FePt/2
nm Fe.sub.3O.sub.4 bimagnetic nanoparticles are shown by TEM. In
FIG. 10C, 8 nm FePt/2 nm Fe.sub.3O.sub.4 bimagnetic nanoparticles
are shown by TEM. In FIG. 10D, 7 nm FePt/1 nm Fe.sub.3O.sub.4
bimagnetic nanoparticles are shown by TEM. In FIG. 10E, 7 nm FePt/3
nm Fe.sub.3O.sub.4 bimagnetic nanoparticles are shown by TEM. The
cores appear darker and shells lighter in the images due to the
large difference in electron penetration efficiency on FePt and
oxides. The shell thicknesses are quite uniform and the main phase
of the shell composes Fe.sub.3O.sub.4 as its structure is confirmed
by XRD and TEM. The FePt/Fe.sub.3O.sub.4 nanoparticle is a
two-phase system consisting magnetically of a hard FePt and a soft
Fe.sub.3O.sub.4 phase. However, the hysteresis loop shows a
single-phase like behavior of the FePt/Fe.sub.3O.sub.4 nanoparticle
and the intimate contact between the FePt core and Fe.sub.3O.sub.4
shell leads to an effective interphase exchange coupling.
Controlling the thickness of the soft phase shell and composition
may lead to increased ductility and deformation during warm
compaction.
[0066] FePt/FeCo nanoparticles are obtained with FePt nanoparticle
seeds with Co(acac).sub.2/Fe(acac).sub.3/polyol precursors in
phenyl ether in the presence of 1,2-hexadecanediol, oleic acid, and
oleylamine, purging the mixture with argon for 30 min at room
temperature and heating to 100.degree. C. for 20 min and then to
300.degree. C. for 90 min., refluxed, and cooled to room
temperature. The black product is precipitated by addition of 20 mL
of ethanol and separated via centrifugation. The product is washed
two or three times using a mixture of hexane (10 mL) and ethanol
(40 mL) and separated via centrifugation. Finally, the product,
FePt/FeCo nanoparticles is redispersed in hexane. The hexane
dispersion of the bricklike nanoparticles is dropped onto a
carbon-coated copper grid for use in transmission electron
microscopy ("TEM"). The copper grid is placed in an alumina boat on
a silica substrate. The boat is then placed in a heating furnace
and purged with a gas mixture (93% Ar+7% H.sub.2) for 20 min. The
samples are heated to the desired annealing temperatures under a
continuous flow of the gas mixture. XRD analysis of the samples
annealed at 500.degree. C. show that the phase transformation of
FePt from chemically disordered fcc to ordered L1.sub.0 begun with
the appearance of FePt L1.sub.0 superlattice peaks. In FIG. 11A, a
TEM of the FePt/FeCo nanoparticle is shown. The room temperature
hysteresis loops of the FePt/FeCo nanoparticles is in contrast to
the as-synthesized FePt hysteresis loops, as shown in FIG. 11B. The
hysteresis loop of the FePt/FeCo nanoparticle shows smooth
"single-phase" demagnetization behavior in second quadrant with a
high remanence ratio (M.sub.r/M.sub.s), which exemplifies
exchange-coupled nanocomposite magnet because of interphase
exchange coupling between the hard and soft phases.
[0067] A typical procedure for obtaining FePt/CoFe.sub.2O.sub.4
bricklike nanoparticles having 8 nm FePt and 8 nm CoFe.sub.2O.sub.4
is the following: Co(acac).sub.2 (0.05 mmol), Fe(acac).sub.3 (0.1
mmol), 1,2-hexadecanediol (10 mmol), oleic acid (10 mmol), and
oleylamine (10 mmol) are mixed in 20 mL of phenyl ether and
magnetically stirred under an argon atmosphere, and 90 mg of 8 nm
FePt seeds (dry powder) is added to the reaction mixture. After the
mixture is purged with argon for 30 min at room temperature, it is
heated to 100.degree. C. for 20 min and then to 200.degree. C. for
60 min., refluxed at 265.degree. C. for 30 min., and finally cooled
to room temperature. The black product is precipitated by addition
of 20 mL of ethanol and separated via centrifugation. The product
is washed two or three times using a mixture of hexane (10 mL) and
ethanol (40 mL) and separated via centrifugation. Finally, the
product, 8 nm FePt/8 nm CoFe.sub.2O.sub.4 bricklike nanoparticles,
is redispersed in hexane. The structure of the nanoparticle
assemblies is examined by transmission electron microscopy ("TEM"),
electron diffraction, and x-ray diffraction ("XRD"). The magnetic
properties are measured by a superconducting quantum interference
device magnetometer.
[0068] Alternatively, the magnetic nanoparticles may also be
synthesized by physical vapor deposition, chemical vapor
deposition, reactive precipitation, sol-gel, microemulsions,
sonochemical processing, supercritical chemical processing,
magnetron sputtering and other methods known in the arts to give
magnetic nanoparticles specific magnetic characteristics or
microstructures. FePt nanoparticles are commonly fabricated using
vacuum-deposition techniques. As deposited, the FePt has a
chemically disordered fcc structure and is magnetically soft.
Thermal annealing is needed to transform the fcc structure into the
chemically ordered fct structure. However, the annealing also
results in nanoparticle aggregation, leading to wide size
distributions. To control the size and narrow the size
distribution, FePt nanoparticles prepared from vacuum-deposition
methods are often buried in a variety of insulator matrices, such
as SiO, Al.sub.2O.sub.3, B.sub.2O.sub.3, or Si.sub.3N.sub.4.
Alternatively, FePt particles can be made by gas-phase evaporation.
Although the average size of these particles can be better
controlled in the improved syntheses, it is still difficult to
disperse them in various liquid media and to use them to form
regular arrays. In contrast with all the physical deposition
processes, solution-phase synthesis offers a unique way for
producing monodisperse nanoparticles, and has been found to be
particularly effective in synthesizing monodisperse FePt
nanoparticles and nanoparticle super-lattices. Water-in-oil ("W/O")
microemulsions use fine microdroplets of the aqueous phase and are
trapped within the assemblies of surfactant molecules dispersed in
a continuous oil phase. The surfactant-stabilized microcavities
provide confinement that limits nanoparticle nucleation, growth,
and agglomeration. Sonochemical processing the acoustic cavitation,
that is, the formation, growth, and implosive collapse of a bubble
in an irradiated liquid, generates a transient localized hot spot,
with an effective temperature of 5000K and a nanosecond lifetime.
The cavitation is a quenching process, and hence the composition of
the particles formed is identical to the composition of the vapor
in the bubbles, without phase separation. High Power Impulse
Magnetron Sputtering ("HIPIMS"), also known as High Impact Power
Magnetron Sputtering and High Power Pulsed Magnetron Sputtering,
("HPPMS")) is a method for physical vapor deposition of thin films
which is based on magnetron sputter deposition. HIPIMS utilizes
extremely high power densities of the order of kWcm-2 in short
pulses (impulses) of tens of microseconds at low duty cycle (on/off
time ratio) of <10%.
EXAMPLES
[0069] The following examples are put forth so as to provide those
of ordinary skill in the art with a complete disclosure and
description of how the compositions, systems, and/or methods
claimed herein are made and evaluated, and are intended to be
purely exemplary and are not intended to limit the scope of
compositions, systems, and/or methods. Efforts have been made to
ensure accuracy with respect to numbers (e.g., amounts,
temperature, etc.), but some errors and deviations should be
accounted for.
Example 1: FePt and Fe.sub.3O.sub.4 Bulk Nanocomposite
[0070] The FePt with face-centered cubic ("fcc") structure and
Fe.sub.3O.sub.4 nanoparticles are mixed at a mass ratio of 8:1 in a
solution before centrifugation, as disclosed in Rong et al. "Bulk
FePt-based nanocomposite magnets with enhanced exchange coupling",
J. Applied Physics, 101: 023908, (2007), herein incorporated by
reference. The dried nanoparticles are heated before compacting
under an Argon atmosphere at 350.degree. C. for 1 hr. to remove
surfactants. The powders are then compacted with a wat
in-compaction press under pressure of 0.5 to 6 GPa for a period of
time and at a temperature. In one embodiment, the pressure is 2.5
or 3.8 GPa for 10 min. at temperatures ranging from room
temperature (about 20.degree. C.) to 600.degree. C. The obtained
bulk samples have dimensions .phi.6 mm.times.1.5 mm and .phi.3
mm.times.1.2 mm for the compaction pressures of 2.5 and 3.8 GPa,
respectively. For comparison, 15 nm L1.sub.0 FePt nanoparticles
prepared by the salt-matrix annealing technique were compacted at
2.5 GPa pressure. The Archimedes method is employed for
measurements of bulk sample density. The morphology and crystalline
structure are characterized by scanning electron microscopy
("SEM"), transmission electron microscopy TEM, and x-ray
diffraction XRD using Cu K.sub..alpha.. radiation. The composition
of the compacted samples are checked by energy dispersive x-ray
("EDX") analysis in SEM. Magnetic properties are measured with
superconducting quantum interference device magnetometer with a
maximum applied field of 70 kOe. For the ideal FePt/Fe composite,
(BH)max=90 MGOe.
[0071] Density
[0072] The bulk samples density .rho..sub.p is dependent on the
compaction temperature ("T.sub.cp") under different pressures. The
density increases monotonously with compaction temperature for both
fcc and L1.sub.0 nanoparticle compacts. The samples prepared at
pressure 3.8 GPa and T.sub.cp) of 600.degree. C. has the highest
density (13.8 g/cm.sup.3) which is about 95% of the full density
value (14.5 g/cm.sup.3 for the FePt/Fe.sub.3Pt composite with 15%
volume fraction of Fe.sub.3Pt phase). Such a high density is a
result of a significant plastic deformation of the nanoparticles at
the applied high pressure. A linear increase in the density can be
observed for the L1.sub.0 particles in the whole studied
temperature range and for the fcc particles in the temperature
range from 20 to about 400.degree. C. This may be explained by the
fact that the yield strength of metallic FePt materials decreases
linearly with temperature in the region between 20 and 800.degree.
C. and an effective lubrication mechanism can occur in the heated
powders. However, this linear increase in density did not lead to
full densification, even when extrapolating the curve to a high
temperature.
[0073] As shown in FIG. 12, for the fcc nanoparticles, the
dependence of .rho..sub.p on T.sub.cp became steeper when
T.sub.cp.gtoreq.400.degree. C. This nonlinear behavior may be
related to the phase transition from the disordered fcc to the
L1.sub.0 ordered structure during which atoms are activated and
become more mobile. The enhanced atomic diffusion promoted
densification of the compacts and results in high density. On the
other hand, the phase transition may also accelerate the formation
of FePt phase in the FePt--Fe3O4 nanoparticle system. Approximately
15% volume fraction of Fe.sub.3Pt phase is determined by using the
method of Rietveld refinement of the XRD pattern, which is
characterization of crystalline materials by the neutron and x-ray
diffraction of powder samples resulting in a pattern characterized
by peaks in intensity at certain positions.
[0074] SEM analysis is performed on the fracture surfaces of the
compacts to characterize morphological changes in the compacted
samples. Typical SEM images of the bulk samples compacted at 20,
400, and 600.degree. C. are shown in FIGS. 13A, 13B, and 13C,
respectively. The 20.degree. C.-compacted samples are quite porous.
With increasing T.sub.cp to 400.degree. C., stripes with thickness
around 10-20 .mu.m are formed. However, some small holes are still
found in the layers. Compacting at 600.degree. C. led to very large
and homogenous areas (>150 .mu.m) in the bulk nanocomposite and
thus the high density. The SEM morphology change is consistent with
the dependence of density on the compacted temperature.
[0075] Phase Transition
[0076] The rapidly increased density at the compaction temperature
higher than 400.degree. C. may be related to the phase transition
of FePt component from fcc structure to the L10 structure. XRD
measurement of the 20, 400, and 600.degree. C.-compacted samples
and the patterns are performed to study the phase transition, as
shown in FIG. 14. The 20.degree. C.-compact mainly consists of fcc
FePt and Fe.sub.3O.sub.4 phases. The compaction at 400.degree. C.
led to the phase transition of FePt component from fcc to L1.sub.0
structure while Fe.sub.3O.sub.4 still exists. With increasing
T.sub.cp to 600.degree. C., the FePt phase is of L1.sub.0 structure
and the Fe.sub.3O.sub.4 disappeared. The quantitative analysis of
the phase content can be made approximately by using the method of
the Rietveld refinement on XRD patterns. The SEM/EDX analysis shows
the decrease of oxygen content from 13.7% for the 20.degree.
C.-compacted sample to 5.0% for the 600.degree. C.-compacted sample
based on big regions. Especially, the oxygen content decreases fast
when T.sub.cp.gtoreq.400.degree. C., which implies that the
nanocrystalline Fe.sub.3O.sub.4 decomposed during the compaction at
a temperature much lower than that reported. The TEM/EDX analysis
based on small regions confirms the existence Fe.sub.3Pt grains in
the 600.degree. C.-compacted samples. The real cause for the
decomposition of Fe.sub.3O.sub.4 may be related to the activated
atoms diffusion during the phase transition of FePt component.
[0077] The degree of phase transition from the disordered fcc to
the ordered L1.sub.0 structure is evaluated in a quantitative way,
with the chemical ordering parameter S calculated by S.apprxeq.0.85
[I.sub.001/I.sub.002].sup.1/2 for the compacts and starting
powders, where I.sub.001 and I.sub.002 are the integrated intensity
of (001) and (002) XRD peaks of the L1.sub.0-FePt phase,
respectively. As shown in FIG. 15, the dependence of S on the
compaction temperature shows clearly that S is almost zero when
T.sub.cp.ltoreq.300.degree. C. while it jumped to 0.9 at
400.degree. C., indicating that the phase transition from fcc to
L1.sub.0 is almost completed for the 400.degree. C.-compacted
samples. For comparison, the dependence of S on the annealing
temperature of the starting powders is also given, where the
annealing time is 10 min, which is same as the compacting time, as
shown in FIG. 15. The phase transition took place at a temperature
(.ltoreq.400.degree. C.) in the compacts lower than the powders and
that reported for the FePt thin films (usually around 600.degree.
C.). The presence of pressure should be responsible for the phase
transition temperature shift. The expedited phase transition under
high pressure is likely associated with the fact that the phase
transition can be described as a compression of the fcc structure
in the direction of the c-axis of the resulted tetragonal phase of
the L1.sub.0 nanoparticle.
[0078] The phase transition is also confirmed by the dependence of
magnetic properties on T.sub.cpl. As shown in FIG. 16, the
dependence of saturation magnetization M.sub.s (measured in an
applied field of 7 T) and coercivity H.sub.C on T.sub.cp. The
samples compacted at relatively low temperatures
(T.sub.cp<400.degree. C.) show nearly zero coercivity, while the
400.degree. C.-compacted sample gave H.sub.C of 10 kOe. The drastic
increase in coercivity originated from the formation of the
L1.sub.0 phase with high magnetocrystalline anisotropy. Since a
higher T.sub.cp led to a higher density of the compacts and thus a
reduced surface effect, the M.sub.s value is increased from 850 to
970 emu/cm.sup.3 with increasing T.sub.cp from 20 to 300.degree. C.
However, the A dropped to 800 emu/cm.sup.3 at T.sub.cp=400.degree.
C., which can be attributed to the phase transition since the
anti-parallel alignment of the polarized Pt spins to the Fe spins
in the L1.sub.0 ordered FePt alloys and thus M.sub.s of the
L1.sub.0 phase is lower than that of the fcc phase. Considering the
FePt/Fe.sub.3O.sub.4 mass ratio of 8:1 in the samples, the decrease
of M.sub.s should be 23% if the fcc phase is completely transferred
to the L1.sub.0 phase, which is in good agreement with the observed
result. The further increase of T.sub.cp results in a fast increase
of M.sub.s, due to the decomposition of magnetite and the formation
of Fe.sub.3Pt phase during the compaction at relatively high
temperature. The mass ratio of FePt/Fe.sub.3O.sub.4 may be
altered.
[0079] Microstructural Characteristics
[0080] X-ray diffraction line-broadening analysis is performed on
the bulks to quantitatively determine the effect of warm compaction
on the microstructure. As shown in FIGS. 17A-17B, the grain size
and retained strain are then deteiiiiined using the Williamson-Hall
analysis method. As shown in FIG. 17A, the grain size is almost
unchanged (around 7 nm) with temperature when
T.sub.cp.ltoreq.300.degree. C. However, the grain size increased
linearly after that to 22 nm when T.sub.cp=600.degree. C. The
pressure has little effect on the grain size; however, the phase
transition promoted grain growth which can be seen from the abrupt
increase in grain size after the temperature is higher than
400.degree. C. The grain size is still under control in the
nanoscale and a higher pressure led to a higher microstrain in the
compacts and the microstrain is reduced with increasing
temperature. As shown in FIG. 17B, when the temperature is
increased to T.sub.cp.gtoreq.400.degree. C., the microstrain is
reduced quickly, since the atomic rearrangement and atomic
diffusion during the phase transition released the strain in the
samples.
[0081] As shown in FIGS. 17A-17B, the grain size of bulk samples
with the post-annealing step (compacted under 2.5 GPa and
post-annealed in forming gas at 500.degree. C. for 1 hr.)
illustrated the effect of heat treatment on the morphology. The
annealing led to the grain growth from 6.5 to 16 nm for the
20.degree. C.-compacted sample, while only from 22 to 25 nm for the
600.degree. C.-compacted sample, implying that the phase transition
is the main reason for the grain growth. This is also evidenced by
the observation that grain size of the L1.sub.0 nanoparticles is
almost unchanged during the annealing. The compacts made by the 15
nm L1.sub.0 nanoparticles show no grain growth up to 600.degree. C.
To confirm, the samples are heated to 1000.degree. C. for 1 hour
and the average grain size is just slightly increased to about 17
nm. The high stability of the L1.sub.0 structured nanoparticles can
be utilized for fabrication of bulk nanocomposite magnets with very
fine and homogenous nanoscale morphology
[0082] As shown in FIGS. 18A, 18B, and 18C, the bright field TEM
images of the bulk samples prepared from fcc nanoparticles under
2.5 GPa pressure at 20, 400, and 600.degree. C., respectively. As
shown in FIG. 18D, the selected area electron diffraction patterns
confirm the phase transition. The TEM images show that grain size
increases with T.sub.cp. The grain size is about 7.+-.3 nm and
14.+-.5 nm for the 20 and 400.degree. C.-compacted samples,
respectively shown in FIGS. 18A and 18B. The grain size and size
distribution are quite small compared to those for bulk materials
fabricated by traditional techniques. The average grain size
depends on the compacting temperature. For the 600.degree.
C.-compacted samples, as shown in FIG. 18C, TEM image shows the
grain size is about 30.+-.10 nm, which is larger than that of XRD
analysis, which is attributed to the existence of large number of
twin grains, as indicated by white arrows.
[0083] Exchange Coupling and Magnetic Properties
[0084] Controlling the grain size of the compacts realizes
intergrain magnetic exchange coupling and achieves high energy
products. The .delta.m=m.sub.d(H)-(1-2m.sub.r(H)) measurements
(Henkel plots) is performed to study the magnetic interactions in
the warm compaction-produced nanocomposite magnets. Here m.sub.d is
demagnetization remanence and m.sub.r is isothermal magnetization
remanence. Both of these values are normalized by the saturation
remanence. Nonzero .delta.m is caused by magnetic interactions
between particles or grains. The positive .delta.m is interpreted
as a sign for magnetic exchange coupling and the negative .delta.m
is a sign of magnetic dipolar interaction. As shown in FIG. 19A,
the .delta.m value for the 600.degree. C.-compacted sample is
positive and much higher than that for the 400.degree. C.-compacted
sample, indicating stronger exchange coupling in the compact
compared to the 400.degree. C.-compacted samples with lower
density. This is also reflected by the shape of the hysteresis
loops, as shown in the inset of FIG. 19A. The remanence ratio
(M.sub.r/M.sub.s=0.63, where M.sub.r, is the remanent
magnetization) of the 600.degree. C-compacted sample is higher than
that of the 400.degree. C. compacted (M.sub.r/M.sub.s=0.58), which
is also consistent with the .delta.m measurement. This effect is
even more pronounced for the L1.sub.0 nanoparticle samples, as
shown in FIG. 19B, where the Henkel plots for 15 nm L1.sub.0
nanoparticles and their compacts. .delta.m for them before
compaction is a large negative value. After the compaction at 2.5
GPa and 200.degree. C., .delta.m changed its sign to positive,
indicating an inter-particle exchange interaction. Annealing of the
compact at 1000.degree. C. for 1 hr. led to further increase in
.delta.m value, which is an enhancement of the exchange coupling.
As the annealing causes almost no obvious grain growth, the strong
increase in .delta.m can be attributed to improvement in interface
conditions upon the high temperature annealing. The hysteresis
loops of the L1.sub.0 nanoparticles is shown in the inset in FIG.
19B and its compacts shows that the enhanced exchange coupling
significantly improves the squareness of the hysteresis loops and
therefore the energy products. A (BH).sub.max of about 15.6 MG Oe
has been obtained based on the measurement from the real
density.
[0085] The magnetic properties can be further improved by a
post-annealing under forming gas (93% Ar+7% H.sub.2) for 1 hr. The
effect of annealing temperature ("T.sub.a") on M.sub.s, H.sub.c,
and (BH).sub.max of the 20, 400, and 600.degree. C. compacts, is
shown in FIGS. 20A, 20B, and 20C, respectively. As shown in FIG.
20A, the M.sub.s of the 20 and 400.degree. C. compacts increased
significantly after the annealing, due to the decomposition of
Fe.sub.3O.sub.4 phase and the formation of Fe or Fe.sub.3Pt with
high magnetization in the reducing atmosphere. The M.sub.s value up
to 1140 emu/cm.sup.3 can be obtained by post-annealing. However,
the M.sub.s of the 600.degree. C.-compacted sample has a very small
change upon post-annealing, which may be due to the early formation
of Fe.sub.3Pt phase during the warm compaction. For all the samples
compacted at the three temperatures, high-temperature annealing
(>600.degree. C.) led to a decrease in M.sub.s, which may be
related to the atomic diffusion between FePt and Fe.sub.3Pt. As
shown in FIG. 20B, the H.sub.c of the 20.degree. C.-compacted
samples increases fast when T.sub.a is higher than 450.degree. C.
which shows that the phase transition from fcc to L1.sub.0 results
in magnetic hardening. For the 400 and 600.degree. C.-compacted
samples, post-annealing did not improve the H.sub.c since the phase
transition have already happened during the wann compaction. In
this case, high-temperature annealing led to an overgrowth in grain
size and therefore deterioration in the magnetic properties. As
shown in FIG. 20C, the (BH).sub.max is up to 16.3 MG Oe, so by
optimizing the post-annealing parameters further improvement of the
(BH).sub.max up to 16.3 MG Oe (based on the real density for
600.degree. C. compact). This (BH).sub.max is significantly higher
than the theoretical limit (13 MG Oe) for the single phase
isotropic FePt magnets.
Example 2: SmCo.sub.5/Fe, NdFeB, and SmCo/Fe Bulk
Nanocomposites
[0086] FIG. 21 shows the comparison of the dependence of density on
the compaction temperature for (a) fcc FePt nanoparticles under
2.5GPa; (b) fcc FePt nanoparticles under 3.8 GPa; (c) L1.sub.0 FePt
nanoparticles; (d) MQ-ribbons; (e) crystallized SmCo.sub.5/Fe
nanocomposite powders; (f) amorphous SmCo/Fe powders. MQ-ribbons
are NdFeB ribbons obtained by rapid quench plus a crushing process.
NdFeB ribbons are compacted at different temperatures under 2.5
GPa. The size of the compacts is about .phi.=6 mm and h=2 mm. The
density can reach 100% at 500-600.degree. C., as shown in FIG. 21.
The XRD patterns of the starting ribbons and 500.degree.
C.-compacted samples that are quite similar. However, the change of
grain size may be determined by a very sensitive XRD machine.
Hysteresis loops are also similar for different compaction
temperatures. For the ideal SmCo/Fe composite, (BH)max=65 MGOe. For
the ideal Sm.sub.2Fe.sub.17N.sub.x.FeCo composite, (BH)max=120
MGOe.
[0087] FIG. 22A is a graph of the XRD patterns of as-milled powders
of SmCo amorphous and very fine Fe nanocrystals. FIG. 22B is a
graph of the XRD patterns of annealed powder of
SmCo.sub.5+.alpha.-Fe nanocomposite warm compacted at 700.degree.
C. for 30 min.
[0088] FIGS. 23A, 23B, 23C, 23D, 23E, and 23F are SEM images of the
surface of the bulk SmCo/Fe samples compacted from the amorphous
powders at different temperatures, room temperature, 200.degree.
C., 300.degree. C., 400.degree. C., 500.degree. C., and 600.degree.
C., respectively. As shown in FIG. 24, FIGS. 23A-23F SEM images
correspond to the amorphous red curve, indicating that density
increases with compaction density. Optionally, the SmCo/Fe samples
may be post annealed under a forming gas (93% Ar+7% H.sub.2) for 1
hr.
[0089] Alternatively, the SmCo/Fe bulk magnets are processed by
ball milling of the SmCo and Fe powders into nanocomposites and
warm compacting the nanocomposite powder particles to form 6-9 mm
samples at 3.5 GPa at <600.degree. C., where the grain size is
in low nanoscale region. FIG. 25A shows the dependence of density
on compaction temperature for amorphous SmCo--Fe powders,
crystallized SmCo--Fe powders, amorphous SmCo powders, and
crystallized SmCo powders. FIG. 25B shows the Vicker hardness (HV)
dependence on compaction temperature for SmCo--Fe amorphous
powders, SmCo--Fe crystallized Fe powders, SmCo amorphous powders,
and SmCo crystallized powders. Amorphous powders are a solid in
which there is no long-range order of the positions of the atoms.
Crystallized or crystalline powders are solids in which there is
long-range atomic order.
[0090] FIGS. 26A, 26B, 26C, and 26D are SEM images of 20.degree.
C., 200.degree. C., 400.degree. C. and 600.degree. C.,
respectively, for compacted bulk nanocomposite SmCo.sub.5/Fe from
crystallized powders.
[0091] The grain size is in low nanoscale region. FIG. 27A shows
the dependence of grain size of SmCo.sub.5 on the compaction
temperature measured from XRD patterns for SmCo--Fe amorphous
powders, SmCo--Fe crystallized Fe powders, SmCo amorphous powders,
and SmCo crystallized powders. FIG. 27B shows the dependence of
grain size of and .alpha.-Fe grains for SmCo--Fe amorphous powders
and SmCo--Fe crystallized Fe powders. FIG. 28A shows the XRD
patterns for SmCo.sub.5+20% Fe ball-milled 4 hours 550.degree.
C..times.30 minutes. FIG. 28B shows the XRD patterns for
SmCo.sub.5+20% Fe ball-milled 4 hours 300.degree. C..times.30
minutes.
[0092] FIGS. 29A, 29B, and 29C are TEM images of 400.degree. C.,
600.degree. C. and 700.degree. C. compacted bulks from the
crystallized nanocomposite SmCo.sub.5/Fe powders, respectively. The
TEM images show the low nanoscale grain size of the SmCo.sub.5/Fe
powders, affected by temperature.
[0093] FIG. 30A is an energy filter TEM image of the SmCo.sub.5/Fe
bulk nanocomposite, where FIG. 30B shows the element distribution
of Co, FIG. 30C shows the element distribution of Fe, and FIG. 30D
shows the element distribution of Sm.
[0094] The SmCo.sub.5/Fe bulk magnet performance showed the first
exchange-coupled nanocomposite isotropic SmCo/Fe magnets. FIG. 31A
is a graph of the dependence of (BH).sub.max on T.sub.cp of the
bulk samples compacted from amorphous and crystallized powders.
FIG. 31B is a graph of the second-quadrant B-H curves of the bulk
nanocomposite and single-phase magnets with maximum energy
products. The energy product enhancement is greater than 70%, as
shown in FIG. 31B the energy product of 16.5 MGOe for
SmCo.sub.5+20% Fe and the single phase counterpart of 9.5 MGOe
energy product.
Example 3: Deformation in Nanocrystalline Metals
[0095] Nanoparticle deformation mechanism and interface atom
diffusion may optimize warm compacting parameters. Utilization of
parallel computing programs and Atomistic Computer Simulations may
detect increased defotmation at and through grain boundaries to
increase the bulk nanocomposite produced by the warm compaction
method, as disclosed in Swygenhoven et al., "Deformation in
Nanocrystalline Metals"; Mats. Today; 9; 5 2006, 24-31, herein
incorporated by reference. Increased deformation at and through the
grain boundaries will increase the density and the magnetization of
the compacts.
Example 4: Naval Applications
[0096] The bulk nanocomposite magnets may be used in axial field
permanent magnet motor/centrifugal pump to improve reliability in
naval applications. Radial field permanent magnet motors have been
demonstrated for quiet undersea vehicle propulsion. The bulk
nanocomposite magnets may also be used for integrated
motor/propulsory in naval machines.
Example 5: Hybrid Cars
[0097] Warm compacted bulk nanocomposites may provide energy
enhancement for hybrid cars for increased energy efficiency.
Example 6: Wind Energy
[0098] Wind energy turbines use permanent magnet generates in the
generator's gear box. The bulk nanocomposites may be used in the
generator gear box for wind energy turbines.
[0099] It will be apparent to those skilled in the art that various
modifications and variations can be made in the embodiments
disclosed herein without departing from the scope or spirit of the
invention. Other embodiments will be apparent to those skilled in
the art from consideration of the specification and practice of the
embodiments disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the invention being indicated by the following
claims.
* * * * *