Scalable Process for Manufacturing Iron Colbalt Nanoparticles with High Magnetic Moment

Abstract

Producing Co.sub.xFe.sub.100-x, where x is an integer from 20 to 95, nanoparticles by: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles, wherein step (c) occurs in an essentially oxygen-free environment. The nanoparticles are annealed at ambient temperatures to yield soft nanoparticles with targeted particle size, saturation magnetization and coercivity. The chemical composition, crystal structure and homogeneity are controlled at the atomic level. The CoFe magnetic nanoparticles have M.sub.s of 200-235 emu/g, (H.sub.c) coercivity of 18 to 36 O.sub.e and size range of 5-40 nm. The high magnetic moment CoFe nanoparticles can be employed in drug delivery, superior contrast agents for highly sensitive magnetic resonance imaging, magnetic immunoassay, magnetic labeling, waste water treatment, and magnetic separation.

Claims

1. CoFe alloy nanoparticles that are produced by a method comprising: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles which have diameters from 5 nm to 40 nm and which have a magnetic saturation (M.sub.s) of from 210 to 235 emu/g, wherein step (c) occurs in an environment that is essentially free of oxygen.

2. The CoFe alloy nanoparticles of claim 1 wherein step (b) comprises preparing an aqueous iron ion solution and an aqueous cobalt ion solution and mixing the aqueous iron ion solution and the aqueous cobalt ion solution to form the second aqueous solution.

3. CoFe alloy nanoparticles of claim 1 wherein the method further comprising recovering the CoFe alloy nanoparticles and annealing the recovered CoFe alloy nanoparticles at a temperature from 20° C. to 25° C. and in an environment that is essentially free of oxygen.

4. The CoFe alloy nanoparticles of claim 1 wherein the CoFe alloy nanoparticles comprises Co.sub.xFe.sub.100-x where x is from 20 to 95.

5. The CoFe alloy nanoparticles of claim 1 wherein the ratio of [M] to [OH.sup.−], where [M]=[Co.sup.2+]+[Fe.sup.3+], ranges from 0.1 to 0.2.

6. The CoFe alloy nanoparticles of claim 1 wherein no CoFe.sub.2O.sub.4 is formed in step (c).

7. The CoFe alloy nanoparticles of claim 1 wherein no CoFe.sub.2O.sub.4 is formed in step (c).

8. The CoFe alloy nanoparticles of claim 1 whereby coprecipitation produces a slurry of CoFe alloy nanoparticles and the method further comprises: (d) washing the slurry of CoFe alloy nanoparticles to remove hydroxide until the slurry reaches a pH of 7; (e) filtering and drying the CoFe alloy nanoparticles; and (f) annealing the washed CoFe alloy nanoparticles in an environment that is essentially free of oxygen.

9. The CoFe alloy nanoparticles of claim 1 wherein the CoFe alloy nanoparticles formed are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

10. The CoFe alloy nanoparticles of claim 1 that are configured as permanent nanocomposite magnets.

11. The CoFe alloy nanoparticles of claim 11 wherein the nanocomposite magnets have diameter of 4 to 10 nm.

12. The CoFe alloy nanoparticles of claim 1 that are configured as soft nanocomposite magnets.

13. The CoFe alloy nanoparticles of claim 12 wherein the soft nanocomposite magnets comprise nanocomposite cores.

14. CoFe alloy nanoparticles that are produced by: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles which have diameters from 4 nm to 8 nm, wherein step (c) occurs in an environment that is essentially free of oxygen.

15. CoFe alloy nanoparticles of claim 14 wherein the CoFe alloy nanoparticles produced are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

16. CoFe alloy nanoparticles that are produced by: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles, wherein step (c) occurs in an environment that is essentially free of oxygen wherein the first aqueous solution and the second aqueous solution are maintained at ambient temperatures between 20° C. to 25° C. and pressure of 24 to 25 Torr under and Ar or Ar/H.sub.2 environment.

17. The CoFe alloy nanoparticles of claim 16 wherein the CoFe alloy nanoparticles produced are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

18. CoFe alloy nanoparticles that are produced by: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles, wherein step (c) occurs in an environment that is essentially free of oxygen.

19. The CoFe alloy nanoparticles of claim 18 wherein the CoFe alloy nanoparticles produced are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

20. CoFe alloy nanoparticles that are produced by: (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles which have a magnetic saturation (M.sub.s) of from 210 to 235 emu/g, wherein step (c) occurs in an environment that is essentially free of oxygen.

21. The CoFe alloy nanoparticles of claim 20 wherein the CoFe alloy nanoparticles produced are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

22. CoFe alloy nanoparticles that are produced by a method comprising (a) providing a first aqueous hydroxide solution; (b) preparing a second aqueous solution containing iron ions and cobalt ions; and (c) depositing measured volumes of the second aqueous solution into the first aqueous solution whereby coprecipitation yields CoFe alloy nanoparticles, wherein step (c) occurs in an environment that is essentially free of oxygen, wherein the CoFe alloy nanoparticles comprises Co.sub.xFe.sub.100-x where x is from 20 to 95, the CoFe alloy nanoparticles have diameters from 4 nm to 8 nm, and the CoFe alloy nanoparticles have a magnetic saturation (M.sub.s) of from 210 to 235 emu/g.

23. The CoFe alloy nanoparticles of claim 22 wherein 1 the method further comprises recovering the CoFe alloy nanoparticles and annealing the recovered CoFe alloy nanoparticles at a temperature from 500° C. to 600° C. and in an environment that is essentially free of oxygen.

24. The CoFe alloy nanoparticles of claim 22 wherein the CoFe alloy nanoparticles produced are selected from the group consisting of Co.sub.20Fe.sub.80, Co.sub.35Fe.sub.65, Co.sub.50Fe.sub.50, Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5.

25. CoFe alloy nanoparticles having the formulation Co.sub.xFe.sub.100-x where x is from 20 to 95, the CoFe alloy nanoparticles have diameters from 4 nm to 8 nm and a magnetic saturation (M.sub.s) of from 210 to 235 emu/g.

26. The CoFe alloy nanoparticles of claim 25 that are configured as permanent nanocomposite magnets.

27. The CoFe alloy nanoparticles of claim 26 wherein the nanocomposite magnets have diameter of 4 to 10 nm.

28. The CoFe alloy nanoparticles of claim 25 that are configured as soft nanocomposite magnets.

29. The CoFe alloy nanoparticles of claim 28 wherein the soft nanocomposite magnets have diameter of 4 to 10 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 depicts an apparatus for co-precipitating CoFe nanoparticles.

[0020] FIGS. 2A, 2B, and 2C are XRD patterns for Co.sub.xFe.sub.100-x (x=35, 50, 85, 90, 95) nanoparticles.

[0021] FIG. 2D are XRD patterns for Co.sub.35Fe.sub.65 and Co.sub.95Fe.sub.5 nanoparticles. Inset shows that grain size increases with increasing annealing temperature.

[0022] FIG. 3A is phase diagram constructed from XRD pattern for Co.sub.xFe.sub.100-x (x=35, 50, 85, 90, 95) nanoparticles.

[0023] FIG. 3B is a phase diagram for CoFe alloys.

[0024] FIG. 4A is an XRD pattern for Co.sub.35Fe.sub.65 nanoparticles.

[0025] FIGS. 4B and 4C are TEM images of as prepared and annealed Co.sub.35Fe.sub.65 nanoparticles respectively.

[0026] FIG. 5A is a graph of M.sub.s vs. atomic % of cobalt in Co.sub.xFe.sub.100-x (x=20, 35, 50, 85, 90) nanoparticles.

[0027] FIG. 5B is a hysteresis loop for Co.sub.35Fe.sub.35 nanoparticles.

[0028] FIG. 6A is a hysteresis loop for Co.sub.35Fe.sub.65 nanoparticles.

[0029] FIG. 6B is a graph of M.sub.s and vs. annealing temperatures for Co.sub.35Fe.sub.65 nanoparticles.

[0030] FIG. 6C is a hysteresis loop for Co.sub.95Fe.sub.5 nanoparticles.

[0031] FIG. 6D is a graph of M.sub.s and H.sub.c vs. annealing temperatures for Co.sub.95Fe.sub.5 nanoparticles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] The invention is directed to fabrication of magnetic nanoparticles (NPs) with high magnetic moment through chemical co-precipitation. Careful control of the alloy NPs' composition and size, with a narrow size distribution, is necessary for achieving high magnetic moment values. M.sub.s value increases substantially with decreasing nanoparticle size. The inventive chemical synthesis technique employs oxygen-free conditions and controls the chemical process at atomic levels to produce precise alloy compositions and size distributions to achieve large M.sub.s and small H.sub.c.

[0033] As shown in FIG. 1, a method of chemical co-precipitation includes preparing an aqueous CoCl.sub.2 and FeCl.sub.3 metal salt solution in container 16. Preferably, a CoCl.sub.2 solution and a FeCl.sub.3 solution are prepared separately using distilled water and the two solutions are added into container 16. A hydroxide solution 12, e.g., NaOH solution, is prepared from distilled water in container 10. Air is removed by vacuum from container 10 through valve 14 until the container pressure is a few mTor before Ar or Ar/H.sub.2 gas is then pumped into chamber 2 of container 30 so that the inner chamber becomes oxygen free. A pair of tubes 22, 24 connect container 16 to container 10 so that the metal salt solution can be added to the NaOH solution 12 in a regulated fashion. The open distal ends of tubes 22, 24 can have the configurations of pipettes. Metering devices 18, 20 such as controlled mini-pumps can be used to deposit drops 26, 28 of metal salt solution into the NaOH. Droplets 26, 28 typically have volumes of 2 ml to 3 ml and are deposited at a typical rate of 2 to 4 drops per sec and preferably 2 to 3 drops per sec. until the metal salt solution is depleted.

[0034] Co-precipitation yields Co.sub.xFe.sub.100-x nanoparticles 30. The size of the nanoparticles in FIG. 1 has been enlarged for illustrative purposes as individual nanoparticles cannot be seen by the naked eye. As co-precipitation proceeds, the NaOH solution becomes turbid and develops into a slurry. Co-precipitation preferably is conducted at ambient temperatures of about 20 to 25° C. and typically at 20° C. The slurry is washed with distilled water to remove the NaOH and filtered. Washing and filtering of the slurry is repeated until a pH of about 7 is reached. The nanoparticles are vacuum dried. Finally, the coprecipitated Co.sub.xFe.sub.100-x nanoparticles are annealed under an Ar or Ar/H.sub.2 environment and a temperature of about 500 to 600° C. and typically at 550° C. for 4 to 6 hours.

[0035] The apparatus of FIG. 1 includes two sources 22, 24 from which the metal salt solution is added to the NaOH solution. Using a plurality of sources reduces the overall reaction time to produce a desired quantity of CoFe alloys. When multiple sources are used, each source should be spaced apart so that their respective co-precipitation reactions do not interfere with each other.

[0036] Introducing measured amounts of the metal salt solution into the NaOH solution, which is an Ar or Ar/H.sub.2 gas oxygen-free, environment, controls the chemical reaction rate and confines the chemical reaction within a limited volume within the NaOH solution 12 in container 10. The technique permits control of the CoFe nanoparticles chemical composition at the atomic level and narrows the particles size distribution.

[0037] In particular, operating in an environment without oxygen prevents the formation of CoFe.sub.2O.sub.4 during the co-precipitation of CoFe alloys; the presence of CoFe.sub.2O.sub.4 would reduce the saturation magnetization (M.sub.s) of CoFe nanoparticles. Moreover, the measured introduction of the metal solution limits the alloy particle size growth, that is, the technique limits particle size growth by slowing the co-precipitation process by depositing limited amounts of metal salt solution, such as by micro-pipetting, which is conducted at room temperature.

[0038] The preferred starting material for preparing the CoCl.sub.2 solution is CoCl.sub.2.6H.sub.2O and the starting material for preparing the FeCl.sub.3 solution is FeCl.sub.3.6H.sub.2O. NaOH is preferred for preparing the hydroxide solution. The co-precipitation reaction that occurs is: CoCl.sub.2+FeCl.sub.3+NaOH.fwdarw.Co.sub.xFe.sub.1-x (x ranges from 0.2 to 0.95)+NaCl+H.sub.2O (Reaction 1). The value of x is determined by the ratio or stoichiometry of the Co.sup.2+ and Fe.sup.3+ in metal salt solution. That is, co-precipitation will generate a CoFe alloy where the proportion of Co to Fe in the alloy is in proportional to the ratio of Co and Fe in the metal salt solution. For example, a metal salt solution with equal molar concentrations of Co.sup.2+ and Fe.sup.3+ will coprecipitate Co.sub.50Fe.sub.50, whereas a metal salt solution with a [Co.sup.2+] to [Fe.sup.3+] ratio of 1 to 2 will generate Co.sub.35Fe.sub.65.

[0039] For example, aqueous solutions of CoCl.sub.2 and FeCl.sub.3 were mixed in a solution of NaOH under the following ratios of [M]/[OH.sup.−]=0.1, 0.2 and 0.4, where [M]=[Co.sup.2+]+[Fe.sup.3+]. The co-precipitation produced a Co.sub.xFe.sub.100-x slurry. The slurry was repeatedly centrifuged and washed with water until pH˜7 and then filtered and dried in airless drying conditions, followed by subsequent low temperature heat treatment to obtain Co.sub.xFe.sub.100-x particle sizes ranging 5-20 nm. To avoid formation of magnetite and CoFe.sub.2O.sub.4 phases and to prevent oxidation of Fe.sup.2+ to Fe.sup.3+, which can potentially decrease the M.sub.s, the particle reduction processes were handled in an airless glove box under Ar or Ar/H.sub.2 protection environment.

[0040] As illustrated in the system shown in FIG. 1, the amount of CoFe NPs that is co-precipitated depends on (1) the concentration of the metal salt solution and its volume in container 16 and (1) the concentration the hydroxide solution and its volume in container 10. According to Reaction 1, if the volumes of the metal salt and hydroxide solutions are the same, for all the metal salt solution to be completely consumed in the co-precipitation, [M]≤[OH.sup.−] where [M]=[Co.sup.2+]+[Fe.sup.3+]. Typically, [M]/[OH.sup.−] ranges from 0.1 to 0.2. The typical molar concentrations of the metal and hydroxide solution is 0.2, which will allow achieving a narrow particles size distribution.

[0041] Co.sub.xFe.sub.100-x (x=35, 50, 85, 90, 95) nanoparticles were prepared by chemical co-precipitation. The specific CoFe alloy made, that is, the value of x, was determined by the ratio of the Co.sup.2+ and Fe.sup.3+ present in the metal salt solution used. The nanoparticles were annealed at different temperatures in Ar/H.sub.2=5% for 8 hrs. FIGS. 2A, 2B, and 2C are x-ray diffraction patterns for the Co.sub.xFe.sub.100-x (x=35, 50, 85, 90, 95) nanoparticles that were prepared. The XRD patterns indicate that the CoFe phase for Co.sub.35Fe.sub.65 and Co.sub.50Fe.sub.50 has a bcc structure and that the CoFe phase for Co.sub.85Fe.sub.15, and Co.sub.95Fe.sub.5 has a fcc structure.

[0042] FIG. 2D is the XRD patterns for Co.sub.35Fe.sub.65 and Co.sub.95Fe.sub.5 nanoparticles. The inset graph depicts the crystal size of Co.sub.35Fe.sub.35 and Co.sub.95Fe.sub.95 nanoparticles, as determined by the Sherrer equation (d=0.9λ/[B cos θ]), as a function of annealing temperature. A similar dependence pattern of crystal size to annealing temperature for Co.sub.50Fe.sub.50, and Co.sub.90Fe.sub.10 nanoparticles was also observed. The data demonstrate that low annealing temperature leads to smaller particle sizes.

[0043] The data in FIGS. 2A, 2B, 2C and 2D show the structural evolution from the bcc structure of the CoFe.sub.2 phase for Co.sub.35Fe.sub.65 and Co.sub.50Fe.sub.50 to the fcc structure of the CoFe.sub.2 phase for Co.sub.85Fe.sub.15 and Co.sub.95Fe.sub.5 nanoparticles, which were annealed at 500° C., 550° C. and 600° C. respectively. The higher annealing temperature led to the formation of more pure bcc CoFe.sub.2, which contributes to M.sub.s=240 emu/g for Co.sub.35Fe.sub.65, and pure fcc CoFe.sub.2, which contributes to M.sub.s=190 emu/g for Co.sub.85Fe.sub.15. The Co-rich Fe alloy particles tends to exhibit reduced magnetostrictive coefficient (λs).

[0044] FIG. 3A is phase diagram constructed from XRD patterns of Co.sub.xFe.sub.100-x (x=35, 50, 85, 90, 95) nanoparticles that were prepared by chemical co-precipitation and annealed at different temperatures in Ar/H.sub.2=5% for 8 hrs. FIG. 3B is a phase diagram of CoFe alloys which was reported in Sourmail, Progress in Material Science, 50 (2005) 816-880. The phase diagram for the Co.sub.xFe.sub.100-x nanoparticles prepared by chemical co-precipitation of the present invention exhibit similar characteristics to that of the prior art CoFe alloys.

[0045] FIG. 4A is the XRD for Co.sub.35Fe.sub.65 nanoparticles containing pure CoFe.sub.2 with a M.sub.s=235 emu/g and particles sizes of 4-10 nm. The nanoparticles were prepared by co-precipitation. These initial nanoparticles were then annealed at 600° C. for 8 hrs, in Ar/H.sub.2. After annealing, the grain sizes increased to 20-60 nm. FIGS. 4B and 4C are transmissions electron microscopy images the initial nanoparticles and the annealed nanoparticles, respectively.

[0046] FIG. 5A is a graph of M.sub.s as a function of cobalt in Co.sub.xFe.sub.100-x (x=20, 35, 50, 85, 90) nanoparticles that were that were prepared by chemical co-precipitation and annealed at 600° C. in an Ar/H.sub.2=5% mixture for 8 hrs. The highest Ms is seen in the Co.sub.35Fe.sub.65 nanoparticles. FIG. 5B is a graph of M v. H and showing a hysteresis loop that was measured at room temperature for the Co.sub.35Fe.sub.35 nanoparticles.

[0047] FIG. 6A is a graph of M v. H and showing a hysteresis loop that was measured at room temperature for Co.sub.35Fe.sub.65 nanoparticles which prepared by co-precipitation and subsequent annealed at 750° C. for 8 hrs, under an Ar/H.sub.2=5% mixture. FIG. 6B is a graph of M.sub.s and H.sub.c vs. annealing temperatures for Co.sub.35Fe.sub.65 nanoparticles which were prepared by co-precipitation and annealed at different temperatures for 8 hrs. under an Ar/H.sub.2=5% mixture.

[0048] FIG. 6C is a graph of M v. H and showing a hysteresis loop that was measured at room temperature for Co.sub.95Fe.sub.5 nanoparticles which prepared by co-precipitation and subsequent annealed at 750° C. for 8 hrs, under an Ar/H.sub.2=5% mixture.

[0049] FIG. 6D is M.sub.s and H.sub.c vs. annealing temperatures for Co.sub.95Fe.sub.5 nanoparticles which were prepared by co-precipitation and annealed at different temperatures for 8 hrs. under an Ar/H.sub.2=5% mixture.

[0050] The high moment CoFe nanoparticles (MNPs) of the present invention can be employed in many applications including drug delivery, superior contrast agents for highly sensitive magnetic resonance imaging (MRI), magnetic immunoassay, magnetic labeling, waste water treatment, and magnetic separation. When incorporated with magnetic hard phase such as SmFeN and NdFeB, MNPs can efficiently sustain high energy product of permanent magnet at high operating temperatures with the advantage of being lightweight, quiet and enhanced magnetic performance. When incorporated with amorphous ZrBCu or SiNbB matrix, MNPs can efficiently enhance magnetic induction Bs of soft nanocomposite magnetic alloys.

[0051] A high temperature permanent nanocomposite magnet was formed by mixing the inventive CoFe nanoparticle and NdFeB nanopowder and then consolidating the mixture to form a bulk nanocomposite magnet. The magnet showed a 200% increase in energy product as compared to a magnet comprising only NdFeB.

[0052] High temperature soft nanocomposite magnets can be made by combining the CoFe nanoparticles with a ZrBCu or SiNbB based amorphous matrix and then consolidating the mixture into a soft magnet. The magnets should exhibit increased induction (Bs).

[0053] The CoFe nanoparticles can be used in MRI as magnetic contrast agents. Currently, only nanocrystalline thin film nanomaterials have been used in MRI. However, thin film nanomaterials oftentimes exhibit poor mechanical properties because of their rough surfaces and cracking films. The contrast agent typically includes the high moment CoFe nanoparticle and a bio-reagent. In preliminary in vivo animal experiments, PLPEG-functionalized FeCo/GC nanocrystals (with M.sub.s=215 emu/g) were injected into rabbits. The experiments confirm the long-lasting positive-contrast intravascular MRI of the blood pool in the rabbit (Seo et al, Nat. Mat. 2006, 5, 971). It is expected that the contrast agent containing inventive CoFe nanoparticles will exhibit even longer-lasting positive contrast intravascular MRI agent properties.

[0054] Another potential use for the CoFe nanoparticles is for integrated diagnosis and therapeutic (photothermal ablation) applications, which was demonstrated by Seo et al, Nat. Mat. 2006, 5, 971.

[0055] The foregoing has described the principles, preferred embodiment and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of present invention as defined by the following claims.