U.S. patent application number 11/214889 was filed with the patent office on 2007-12-06 for magnetic field assisted synthesis of magnetic nanoparticles.
This patent application is currently assigned to UNIVERSITY OF DELAWARE. Invention is credited to John Q. Xiao, Yuwen Zhao.
Application Number | 20070277649 11/214889 |
Document ID | / |
Family ID | 38788583 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070277649 |
Kind Code |
A1 |
Xiao; John Q. ; et
al. |
December 6, 2007 |
Magnetic field assisted synthesis of magnetic nanoparticles
Abstract
A process for the production of magnetic nanoparticles having
uniform size and shape in which a magnetic metallic substance
selected from magnetic transitional metals, their alloys, and
intermetallic alloys with non-magnetic metals, is dissolved in a
non-aqueous medium, reacted with a reducing agent, in solution and
the reaction mixture is subjected to an external magnetic
field.
Inventors: |
Xiao; John Q.; (Newark,
DE) ; Zhao; Yuwen; (Newark, DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
1875 EYE STREET, N.W.
SUITE 1100
WASHINGTON
DC
20036
US
|
Assignee: |
UNIVERSITY OF DELAWARE
Newark
DE
|
Family ID: |
38788583 |
Appl. No.: |
11/214889 |
Filed: |
August 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60606347 |
Sep 1, 2004 |
|
|
|
Current U.S.
Class: |
75/348 |
Current CPC
Class: |
H01F 1/0054 20130101;
B22F 2998/00 20130101; B22F 2999/00 20130101; B22F 9/24 20130101;
B82Y 30/00 20130101; B22F 1/0018 20130101; B22F 2998/00 20130101;
B82Y 25/00 20130101; B22F 2999/00 20130101; B22F 2202/05 20130101;
B22F 9/24 20130101 |
Class at
Publication: |
075/348 |
International
Class: |
B22F 9/24 20060101
B22F009/24 |
Goverment Interests
GOVERNMENT LICENSE RIGHTS
[0002] The United States Government has rights in this invention as
provided for by U.S. Defense Advanced Program Agency (DARPA) Grant
No. F33615-01-2-2166.
Claims
1. A process for producing magnetic nanoparticles of uniform shape
and size comprising: a. forming a solution in a non-aqueous medium
of a magnetic substance selected from the group consisting of
magnetic transition metals, their salts and alloys and
intermetallic alloys of said magnetic transition metals with
non-magnetic metals; b. adding a reducing agent to the solution to
form a reaction mixture, and c. applying an external gradient
magnetic field to the reaction mixture.
2. A processing according to claim 1 in which the non-aqueous
medium is a lower alcohol.
3. A process according to claim 2 in which the alcohol is
ethanol.
4. A process according to claim 1 in which the reducing agent is a
borohydride.
5. A process according to claim 3 in which the magnetic substance
is cobalt.
6. A process according to claim 1 in which the magnetic substance
is cobalt.
7. A process according to claim 1 in which the magnetic substance
is a cobalt alloy with a non-magnetic metal.
8. A process according to claim 1 wherein the magnetic substance is
an alloy of cobalt and another magnetic transition metal.
9. A process for producing magnetic nanoparticles of uniform size
and shape comprising: a. dissolving a cobalt salt in ethanol; b.
dissolving a borohydride compound in ethanol; c. adding the
borohydride solution to the cobalt salt solution so as to cause a
reduction reaction in the mixture; d. applying an external magnetic
field to the reaction mixture.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/60,347 filed Sep. 1, 2004, the content of
which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0003] Magnetic nanoparticles are known to be very important in a
wide variety of applications, such as ultrahigh density magnetic
data storage, catalysts, drug delivery, cell separation,
diagnostic, hypothermia for cancer treatment, inductive bonding,
ferrofluids and magnetic sensors. If such magnetic nanoparticles
are to be used with the greatest effectiveness, it is crucial that
they be produced with uniform size and shape and in a process that
can be easily scaled up for large quantity production.
[0004] Existing techniques for the production of magnetic
nanoparticles are problematic in that they are very costly,
impractical for mass production or involve the use of undesirable
toxic materials. For example, monodispersed nanoparticles of
transition metals and their alloys may be produced by a polyol
process, but it requires the use of expensive noble metals as seeds
in order to obtain the desired small size.
[0005] Micro-emulsion or reverse micromulsion methods have produced
acceptable magnetic nanocrystals but they are unsuitable for large
quantity production.
[0006] Thermal decomposition methods are also known to produce
magnetic nanoparticles but the organic metallic compounds used in
this process, such as iron pentacarbonyl [Fe(CO)s], dicobalt
octacarbonyl [CO.sub.2(C).sub.8], nickel tetracarbonyl
[Ni(CO).sub.4], are expensive, unstable and very toxic. Thus, such
methods are not conducive for large quantity production.
[0007] This invention avoids these problems and produces large
quantities of uniformly-sized magnetic nanoparticles by controlling
the growth of the emerging nanoparticles as they are formed through
the use of an external magnetic field.
SUMMARY
[0008] In this invention, a novel process is provided for producing
nanocrystals of uniform shape and size from magnetic transition
metals, salts thereof, alloys of said metals and alloys of said
metals with non-magnetic metals.
[0009] The process comprises essentially dissolving the selected
magnetic transition metal, salt or alloy thereof in a non-aqueous
solution and reducing the dissolved transition metal, salt or alloy
in the presence of an external magnetic field.
[0010] In a preferred embodiment, borohydrides are employed as the
reducing agent and the transition metal or alloy is dissolved in
ethanol.
[0011] By performing the reduction reaction in a non-aqueous medium
one is able to avoid the formation of transition metal borides
which have much weaker ferromagnetism than pure transition metals
and their alloys. By employing an external magnetic field in the
reaction solution, one is able to control the nucleation, growth
and size of the magnetic nanoparticles so as to achieve very
uniform nanoparticles in large quantity. Any non-magnetic particles
in the solution will not be attracted by the external magnetic
field and may be separated out.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic drawing of the experimental setup
utilized in the example described herein.
[0013] FIG. 2 presents TEM graphs (a) and (b) show size
distribution and shape and (c) to demonstrate selected area
electron diffraction pattern.
[0014] FIG. 3 shows an XRD pattern of Co nanocrystals
as-synthesized (red line) and subsequently annealed at 600.degree.
C. in Ar atmosphere for 1 hour (blue line). Vertical bars indicate
the diffraction peak positions of fcc Co from JCPDS number
15-0806.
[0015] FIG. 4 is a hysteresis loop of as-synthesized Co
nanocrystals (a, blue line) and subsequently annealed at
600.degree. C. (b, blue line) for 1 hour and at 700.degree. C. (b,
red line) for 3 hours in H2 atmosphere.
DETAILED DESCRIPTION
[0016] The magnetic nanoparticles produced according to the process
of this invention are formed from magnetic transition metals such
as iron, cobalt and nickel, salts thereof, their alloys and
intermetallic compounds such as Co.sub.3Pt, CoPt, CoPt.sub.3,
Fe3Pt, FePt and FePt.sub.3.
[0017] It is important to this invention that the metals or their
alloys which form the intended magnetic nanoparticles are reduced
in a non-aqueous solution, preferably a lower alcohol such as
methanol, ethanol, propanol, isopropanol and butanol. Ethanol is
especially preferred.
[0018] Any of the known reducing agents may be used in the process
of this invention, although a strong borohydride such as NaBH.sub.4
or KBH.sub.4 is preferred.
[0019] It is crucial to the formation of magnetic nanoparticles of
uniform size and shape that the reaction solution be subjected to
an external gradient magnetic field so as to adjust the dwelling
time of the newly-formed magnetic nanoparticles in the reaction
site, thus controlling their growth.
[0020] The materials produced by this invention may be used in
exchanged coupled soft and hard magnets, ultrahigh density data
storage media, catalysts, biological and medical applications such
as cell separation, targeted drug delivery and hyperthermia
treatment of tumors and infectious diseases.
EXAMPLE
[0021] CoCl.sub.2 and NaBH.sub.4 ethanol solutions were first
prepared by dissolving appropriate amount of CoCl.sub.2 and
NaBH.sub.4 into ethanol with vigorous stirring. The molar ration of
NaBH.sub.4 over CoCl.sub.2 was purposely made larger than 2 to
ensure complete reduction of CoCl.sub.2. The CoCl.sub.2 ethanol
solution was then put on top of a mineral oil layer which has a
density larger than the above solution. NaBH.sub.4 ethanol solution
was added into CoCl.sub.2 ethanol solution in a drop-like manner by
using a dropping funnel or better using an ultrasonic atomizer that
can give rise to small droplets of .about.40 .mu.m. According to
Jackelen et al. [A. M. L. Jackelen, M Jungbauer, G. N. Glavee,
Langmuir 1999, Vol. 15, PP 2322-2326.], the following chemical
reactions take place instantaneously when the NaBH.sub.4 ethanol
solution is added:
CoCl.sub.2+2NaBH.sub.4.fwdarw.Co+H.sub.2+B.sub.2H.sub.6+2NaCl
B.sub.2H.sub.6+6CH.sub.3CH.sub.2OH.fwdarw.2B(OCH.sub.2CH.sub.3).sub.3+6H.-
sub.2
[0022] The magnetic Co particles were attracted by a permanent
magnet placed under the reaction beaker, which create a gradient
magnetic field, (see FIG. 1) first into the interface between the
oil and ethanol phases and finally into the oil phase. In this way,
the particles are immediately taken away from the reaction site
where BH.sub.4.sup.- ions are continually supplied. After the
reaction was completed, the supernatant solution and the oil were
discarded. The slurries were washed by ethanol, acetone and
de-ionized water to thoroughly remove the residual oil and NaCl
formed during the reaction, and finally washed by acetone again to
remove water.
[0023] The resulting nanoparticles are shown in the drawing to have
uniform size and shape. FIGS. 2a and b show the TEM graphs of the
as-synthesized Co nanocrystals and confirm such a result. Very
homogeneous particles with near perfect spherical shape are clearly
seen in FIG. 2b, a histogram made using 250 randomly selected
particles shows that the average particle size is around 4.7 nm
with standard deviation of 1.6 nm. Since the as-prepared particles
are small, the XRD pattern shows only a very broad peak [FIG. 3,
bottom trace] corresponding to the diffraction line of face
centered cubic (fcc) Co. After annealing the as-prepared particles
at 600.degree. C. for 1 h with argon flow, all diffraction lines of
fcc Co appear but no Co--B alloy diffraction lines are detected
[FIG. 3, upper trace]. In contrast to the present invention, this
is usually found when the reduction is carried out in an aqueous
solution. Therefore, by the method described here, one can
fabricate pure Co nanoparticles. Although the XRD pattern for the
as-prepared .mu.Co nanoparticles lacks fine structure, the
crystallinity was evidenced by the TEM associated electron
diffraction pattern [FIG. 2(c)] indicating that the particles are
well-crystallized fcc Co.
[0024] In FIG. 4(a), the M-H loop (solid triangles and line) for
the as-prepared Co nanoparticles is plotted and measured at RT,
showing typical superparamagnetic behavior. The magnetization curve
can be fitted to a Langevin function with the summation of two size
components [FIG. 4(a), dotted line; fitting parameters are
indicated on the figure]
M=M.sub.1.times.[coth(.mu..sub.1H/k.sub.BT)-k.sub.BT/.mu..sub.1H]
+M.sub.2.times.[coth(.mu..sub.2H/k.sub.BT)-k.sub.BT/.mu..sub.2H]
where M.sub.i and .mu..sub.i (i=1, 2) are the saturation moment and
the effective moments of a unit magnetic cell of each size
component, respective and k.sub.B is the Boltzmann constant.
[0025] The TEMP graph shown in FIG. 2(b) indicates that the
particles are almost perfectly spherical.
[0026] The zero-field cooled (solid triangles and dotted line) and
5 T-field cooled (solid circles and line) M-H loops (FIG. 4(b)'
measured at 5 K nearly repeat each other, implying that the
oxidation of particles were minimized.
[0027] While there is shown and described certain specific
structures embodying the invention, it will be manifest to those
skilled in the art that various modifications and rearrangements of
the parts may be made without departing from the spirit and scope
of the underlying inventive concept and that the same is not
limited to the particular forms herein shown and described.
* * * * *