WO2009117718A1 - Direct chemical synthesis of rare earth-transition metal alloy magnetic materials - Google Patents

Abstract

A direct chemical synthesis scheme for the fabrication of isotropic and anisotropic rare earth (RE) -transition metal (TM)- based permanent magnets is disclosed. The method according to the invention permits the synthesis of, e.g., particles, films and coatings with controlled particle sizes, shapes, compositions and dispersion in one reaction process. The resulting particles have desirable hard magnetic properties such as high magnetic saturation induction and coercivity at room temperature.

Description

DIRECT CHEMICAL SYNTHESIS OF RARE EARTH-TRANSITION METAL ALLOY MAGNETIC MATERIALS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 61/070,136, entitled DIRECT CHEMICAL SYNTHESIS OF RARE EARTH (RE) - TRANSITION METAL (TM) ALLOY MAGNETIC MATERIALS AND METHOD OF PRODUCING THE SAME, filed March 20, 2008, the whole of which is hereby incorporated by reference herein .

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT NONE

BACKGROUND OF THE INVENTION

The superior magnetic properties of rare earth (RE)- transition metal (TM) alloy materials in general and, most particularly, the superior properties of Sm-Co based magnets, especially their excellent high temperature stability and high maximum energy product at temperatures well above ambient, have led to an exciting variety of useful applications. Additionally, RE-TM alloy products that include metalloids, e.g., Nd-Fe-B alloy based magnets, have obtained commercial success based largely upon their outstanding performance for room temperature applications .

However, Sm-Co magnets continue to play a dominant role in some critical applications, such as in shaping and controlling electromagnetic and particle beams in cathode-ray tubes, traveling wave tubes, magnetrons, klystrons, ion pumps, and cyclotrons; in inertial devices for accelerometers and gyroscopes; in miniature high performance motors for power tools used in commercial and specialized medical applications; in permanent magnet motors and generators for aircraft engines, hybrid cars, and alternative energy technologies such as wind turbines; and in providing bias fields for microwave electronic devices such as circulators, isolators, phase shifters, filters, etc .

The fabrication and, in particular, the direct chemical synthesis of Sm-Co-based nanomagnets exhibiting large coercivity and high magnetic moment has been a long standing goal for the materials and engineering research communities owing to the potential applications of these materials in high-performance permanent magnets. SmCo₅ and Sm₂Cθi7 are the most important hard magnetic materials in the Sm-Co alloy system. These alloy materials adopt a hexagonal close-packed (hep) structure with Co and Co+Sm present in alternating layers along the crystallographic c-axis. The easy magnetization direction of Sm- Co is aligned along the c-direction of the lattice, and the magnetocrystalline anisotropy constant ranges from 1.1-2.0 x 10⁸ erg cm^"3; these values are among the highest known for hard magnetic materials. Furthermore, the alloys in this system exhibit a very high Curie temperature ( T_c ~ 1020 K), which makes this system superior to other classes of permanent magnetic materials, such as FePt ( T_c ~ 750 K) and Nd₂Fe_I4B ( T_c ~ 585 K), for high-temperature applications.

However, as with other rare earth metal materials, metallic Sm-Co alloy nanoparticles are prone to rapid oxidation. This chemical instability has made the chemical synthesis of, e.g., nanostructured SmCo₅ and Sm₂COi₇ extremely difficult. For example, ball milling and melt spinning, the two standard physical methods used at the present time in the fabrication of nanostructured SmCo magnets, provide only limited control of the sizes of the final magnetic grains and/or particles. Solution- phase chemical synthesis approaches have been applied to prepare monodispersed magnetic nanoparticles and have recently been extended to the synthesis of SmCo₅ nanoparticles by coupling the polyol reduction of samarium acetylacetonate, i.e., Sm(CH₃COCHCOCH₃) ₃, with the thermal decomposition of Co₂(CO)₈ (Gu et al., 2003). Although the molar ratio of Sm/Co in the two- component nanoparticles can be tailored by this method to reach 1:5, there is no conclusive evidence that hep-structured SmCo₅ is formed or that hard magnetic properties are obtained for the resultant alloy nanoparticles.

Recently, nanocrystalline SmCo₅ and SmCo₅-Fe were synthesized by the reductive annealing of core/shell-structured Co/Sm₂0₃ and Fe₃O₄. (Hou, Xu et al . , 2007; Hou, Sun et al., 2007). SmCo_x nanoparticles also have been synthesized by using Au as an additional element in the composition (Matushita et al . , 2007). These authors report that, otherwise, the Sm(III) ions are not completely reduced and that, without this element present, the prepared samples will easily become oxidized, as shown from their X-ray diffraction pattern. As another example, Sm-Co alloy nanoparticles have been synthesized from Sm-oleate and Co-oleate complexes followed annealing (Shin et al . , 2007). However, the coercivity of these complexes was only about 458 Oe at room temperature, even after annealing. This value is far smaller than values required for the targeted hard magnet applications. These advances still require expensive multistep processes to realize the final hard magnetic material products. For this reason, a single-step method for direct chemical synthesis of high coercivity (H_c) and crystalline rare earth - transition metal materials, e.g., alloy materials, is highly sought after for cost-effective industrial processing. Such a synthesis method is the focus of the invention described herein. BRIEF SUMMARY OF THE INVENTION

The invention is directed to a method of making a rare earth-transition metal magnetic material, the method including the steps of combining a rare earth metal salt and a transition metal salt in a solvent to form a mixture, wherein the solvent is a pure solvent or a solvent solution and wherein the pure solvent or solvent solution has the properties of both a reducing agent and an oxidation prevention agent; causing precipitation of particles of a magnetic material; and collecting precipitating particles from the mixture. Preferably, said precipitating particles are nanoparticles . Alternatively, the precipitating particles are microparticles . In another embodiment, the particles are precipitated in the form of a film or a coating on a substrate.

The metal salts in the starting precursor materials comprise a combination of (i) one or more rare earth based salts, e.g., RE-chloride, RE-sulphate, RE-acetylacetonate, RE- acetate, RE-hydroxide, RE-nitrate, and/or RE-flouride, and (ii) one or more transition metal (TM) based salts, e.g., TM- chloride, TM-sulphate, TM-acetyl acetonate, TM-acetate, TM- hydroxide, TM-nitrate and/or TM-flouride. Exemplary reducing agents are polyalcohol, polyol, glycol and hydroxides, such as NaOH and KOH. Long chain alkyl diols and alkyl alcohol can be used as a co-surfactant or as co-reducing agents to facilitate particle growth and separation (i.e., dispersion).

Further processing methods for the particles produced by the method of the invention to produce the desired RE-TM nanoparticles and films/coatings include, but are not limited to, the microwave polyol process, ultrasonication, dip pen lithography (and other direct write processes such as ink jet) , electroless deposition, laser based (or other photon-induced or assisted processes) synthesis, spin coating techniques based films/coatings and polyol derived films/coatings on various substrates, including: metals, oxides, superconductor, and/or semiconductors. The advantages of the invention described herein include: (i) the method of the invention is a direct, one-step, synthesis process resulting in SmCo metallic particles (and/or films/coatings) that, without requiring further external heat treatment, possess desirable hard magnetic properties such as high saturation induction and high coercive field strengths;

(ii) the process can readily be scaled to industrial process levels; (iii) the process is dependent upon precursor materials and solutions that are readily available, cost-effective, and permit recycling, thus making this a "green" technology; and

(iv) the process according to the invention can readily be altered to control composition, structure, size, shape, and dispersion of resulting particles and particulate/granular coatings .

Contemplated applications additional to those known in the art include using the magnetic material forned by the process of the invention as a biolabelling agent and/or in targeting and carrier applications in biomedical technologies.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which:

Fig. 1 shows a representative X-ray diffraction pattern, using Cu ka radiation, illustrating the presence of the predominant SmCo phases, SmCo₅ (a=0.4991 nm; c=0.3977 nm) and Sm₂COi₇ (a=0.8367 nm; c=0.8192 nm) (+/- 0.002 nm for a and c of both phases) . The sample was processed using the following conditions: Samarium acetate hydrate and Cobalt acetate tetrahydrate at 1: 5 wt . % ratio with a reaction temperature of 558 K for 4 hours; PVP (mw ~ 40,000) at 0.54 gram; Fig. 2 shows representative high resolution transmission electron microscopy images and selected area diffraction. Fig. 2 (a) ahows a low and Fig. 2 (b) shows a high magnification TEM image of the SmCo nanoblades, Fig. 2 (c) is an HRTEM image showing that the growth direction of the blade is [100] (perpendicular to the [200] planes) and that one of the surface planes parallel to the growth direction is the [001] plane. Fig. 2 (d) is the electron diffraction pattern from the nanoblade shown in Fig. 2 (c) , indicating that the blade is oriented along the [010] zone axis and is consistent with the HRTEM image, showing the SmCo₅ phase;

Fig. 3 shows hysteresis loops of air-stable SmCo nanoblades measured at room temperature and at 10 K. The intrinsic coercivity and the magnetization of the nanoblades are 6.1 kOe and 40 emu/g at room temperature;

Fig. 4 (a) shows hysteresis loops at room temperature for exchange coupled (or exchange spring) SmCo+Fe nanoparticles depicting the effect of increasing amounts of metallic Fe phase co-existing with the SmCo phase (s). The shape of the loops (lacking a pronounced "knee") suggests exchange coupled phases. Fig. 4 (b) lists the coercivity and saturation magnetization values for the three samples plotted in Fig. 4 (a) ;

Fig. 5 shows X-ray diffraction (Cu ka radiation) patterns illustrating the effect of modifying the molar concentration ratio of Sm: Co. The predominant SmCo phases are SmCo₅ and Sm₂COi₇. Samples were processed using the following conditions: Samarium acetate hydrate and Cobalt acetate tetrahydrate with the indicated molar ratio at a reaction temperature of 558 K for 4 hours; PVP at 0.54 gram;

Figs. 6(a)-6(d) are TEM images showing the effect of shape control on the RE-TM alloy nanoparticles. The predominant SmCo phases are SmCo₅ and Sm₂COi₇. Samples were processed with the following additives: NaOH 0.6 g and PVP (mw: 40,000) = 0.26 g. Fig. 6 (a) shows that Samarium acetyl acetonate and Cobalt acetyl acetonate precursors at the standard conditions produce spherical particles, Fig. 6 (b) shows that Samarium acetate hydrate and cobalt acetate tetrahydrate at a reaction temperature of 473 K and a reaction time of 2 hours in tetraethylene glycol produce rod-like particles, Fig. 6 (c) shows that the same RE-TM metal salts at a reaction temperature of 558 K for 4 hours produce blade-like particles and Fig. 6 (d) shows a large number of of the uniform blade-like nanoparticles synthesized as shown in Fig. 6 (c) ;

Fig. 7 shows typical room temperature hysteresis loops of the as-synthesized Sm_xCθioo-x nanoparticles as a function of the molar concentration ratios given in Fig. 5. The loops show single-phase-like behaviour, indicating effective exchange coupling between SmCo₅ and Sm₂COi₇ phases; and

Fig. 8 shows room temperature magnetization and coercivity plots for the Sm_xCθioo-x nanocomposites of Fig. 7 as a function of Sm molar concentration. All the data were collected at room temperature on a superconducting quantum interference device (SQUID) magnetometer with field up to 6 T.

DETAILED DESCRIPTION OF THE INVENTION

The entirety of U.S. Provisional Application No. 61/070,136, entitled DIRECT CHEMICAL SYNTHESIS OF RARE EARTH (RE)-TRANSITION METAL (TM) ALLOY MAGNETIC MATERIALS AND METHOD OF PRODUCING THE SAME, filed March 20, 2008, from which this application claims priority, is hereby incorporated by reference herein.

The method of the invention is a direct chemical synthesis scheme for the fabrication of isotropic and anisotropic rare earth (RE) -transition metal (TM) -based permanent magnets. The method according to the invention permits the synthesis of, e.g., particles, films and coatings with controlled particle sizes, shapes, compositions and dispersion. The resulting particles have desirable hard magnetic properties such as high magnetic saturation induction and coercivity at room temperature. Thus, the method of the invention extends the range of decomposition/reduction chemistry to permit the production of RE-TM permanent magnetic materials and exchange coupled (or exchange spring) magnetic materials with alloy as well as core-shell structure and with control over particle size (e.g., between 2 and 10,000 nm) , particle shape (e.g., spheroids, wires, rods, cubes, blades, etc.), and coercive fields (e.g., up to at least 20 kOe at room temperature). Saturation induction can fall between, e.g., 4 kG to 20 kG for both the non-exchange and exchange coupled particles. The structure of the particles is either a core-shell of hard and soft magnets or an alloy of the same with strong exchange coupling .

The present invention relates to the chemical reduction of metal salts to form RE-TM alloy particle materials, particularly nanoparticle materials, with control of composition, structure, size, shape, and dispersion n. Exemplary RE metals are elements of atomic number 57 through 71 (Sm, La, Ce, Pr, Pm, Nd, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and commonly occurring isotopes of the same; and exemplary TM metals are elements of atomic number 24 through 28 (Cr, Mn, Fe, Co, and Ni) and commonly occurring isotopes of the same. Most practical applications will see the raw powder resulting from the invention discussed here being compacted and sintered. The magnetic properties of the resulting (nano) particles and their assemblies can be tuned from superparamagnetic to ferromagnetic behavior (based predominantly on particle size and composition) , with coercive fields up to at least 20 kOe at room temperature.

In addition to RE-TM intermetallics (SmCo₅, Sm₂COi₇, and Nd₂Fe_I4B among others) , the process described herein allows for excess transition metals, such as Fe, Co, Ni, Mn, Cr, and their alloys, to be present and coexist with the RE-TM intermetallic (s) . This allows for the realization of exchange coupled (or exchange spring) magnet systems, for example, SmCo+Fe, SmFe+Co, SmCo+CoFe, SmCo+Co, NdFeB+Fe, NdFeB+CoFe, NdFe+Co, etc. (either in an alloy form or core-shell structure form) . Such exchange coupled magnet systems have been predicted to have permanent magnet figures of merit that far exceed the RE-TM intermetallics phases (without excess TM phase (s) ) .

X-ray diffraction images of the resulting particles reveal diffracted peaks that appear at 2-theta (in degrees) values that are consistent with the Sm₂COi₇ and/or SmCo₅ phases. Oother phases present include, in some instances, metallic Fe and Co or FeCo alloys. However, peak amplitudes and relative intensities, coupled with shifts in 2-theta positions, do not allow for absolute confirmation of the identification of these SmCo phases. While not being bound by any theory, it can be stated with confidence from Rietveld full profile fitting results that: (i) there are no oxide phases present, including TM-oxides and/or RE-oxides within the uncertainty of the measurements and analyses (presumably <3 %) ; (ii) the ratio of RE : TM ions is commensurate with the nominal chemical compositions of precursor solutions; and (iii) although no confirmation of the 2:17 and 1:5 phases can be presented, the results are consistent with a chemically disordered equivalent phases or solid solutions. However, a new SmCo phase cannot be ruled out.

The present invention utilizes the reduction of metal salts to produce RE-TM alloy materials. The reduction can be a polyol reduction of a mixture of one or more types of rare earth metal salt (such as RE-chlorides, RE-sulphates, RE-acetyl acetonates, RE-acetates, RE-hydroxides, RE-nitrates and RE- flourides) and one or more types of transition metal salt (such as TM-chlorides, TM-sulphates, TM-acetylacetonates (i.e., CH₃COCHCOCH₃), TM-acetates, TM-hydroxides, TM-nitrates and TM- flourides) . More than one RE-salt and/or more than one TM-salt will result in compositions such as (RE1-RE2) - (TM1-TM2) . Exemplary polyols are the polyalcohols and the glycols. Alternatively, the reduction is carried out in another solvent such as an ether or an amine, e.g., phenyl ether, octylether and trialkyl amine, in which the RE-TM salts will reduce with the help of additional reducing agents such as compounds containing diol groups (for example, 1, 2-hexadecane diol) . The RE-salts and TM-salts are usually single metal-salts. However, mixed metal salts, e.g., REi-RE₂ or RE-TM-salts, could also be used. The RE- and TM- salt solutions are generally introduced to the solvent at the same time. For example, since Sm has a very high negative reduction potential, i.e., -2.30 V, it is important to co-reduce Sm and Co metal salts to successfully form SmCo nanoparticles . However, adding the RE salt first to the solvent at a particular temperature and then the TM salt at a different temperature is also contemplated.

The process reaction can be carried out at temperatures ranging, e.g., from room temperature to over 600 K, as long as the temperature is kept below the boiling point of the solvent used. Energy may be added to accelerate the reaction including the nucleation and growth processes. Energy can be introduced to the reaction vessel in the form of microwaves, sounds waves via ultrasonicators, or light of any wavelength and intensity.

The composition, size, shape, structure, and dispersion are controlled by, e.g., the (i) initial molar ratio of all the reactants (e.g., of all of the metal salts and the reducing agents in solution) , (ii) reaction time, (iii) RE/TM metal salt ratio, (iv) heating rate, (iv) reaction temperature, (v) reaction time, (vi) type of solvent, (vii) effect of seeding, (viii) addition of other elements, e.g., as catalysts, (ix) mixture of various polyols in different ratios, and (x) ratio of surfactants (such as olelamine, oleic acid and carboxylic acids) or capping agents (such as poly- (N-vinylpyrrolidone) (PVP) or polyethyleneimine (PEI)) to the RE-TM salts.

Nanosized RE-TM particles have been produced by controlling the molar concentration of the metal salts. For example, with a total salt concentration of 0.005 to 0.2 M, e.g., at the conditions described herein, the end product of the reaction generally will be in the nanoparticle range. If the concentration is greater than 0.2 M, then the final product will be in the micron sized range.

The particles are protected from aggregation by a combination of a long chain carboxylic acid, i.e., RCOOH, wherein R=C_n where n is 8 or larger, and a long chain primary amine, RNH₂, wherein R=C_n where n is 8 or larger. The particles are easily dispersed in alkane, arene and chlorinated solvents and purified by precipitation through the addition of alcohol. Deposition of the alkane solution of the alloy particles on a solid substrate forms a smooth particulate/granular film and/or coating. Feasible substrates vary widely and may be metals, oxides, superconductors, or semiconductors, common substrates include SiO₂, Si, Si₃N₄ or glass. Exemplary RE-TM alloy particles as described herein, in particular SmCo, show a hexagonal close packed (hep) structure and are magnetically hard. A secondary heat treatment in the range of 673-1173 K was shown to result in a high energy product (i.e., the figure of merit for most hard magnet materials) , and it is expected that particles resulting directly from the method of the invention described herein may require compaction and/or secondary thermal treatments to realize desired magnetic hard properties for specific applications .

The present invention provides a method of producing a magnetic metal powder composed of magnetic metal particles of a substance represented by the general formula (1):

[REχTM_γ] (1),

where RE is one or a combination of elements of atomic number 57 through 71 (Sm, La, Ce, Pr, Pm, Nd, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and commonly occurring isotopes of the same; and exemplary TM metals are elements of atomic number 24 through 28 (Cr, Mn, Fe, Co, and Ni) and commonly occurring isotopes of the same .

For some applications, the substance of magnetic metal particles produced by the method of the invention can be represented by the general formula (2):

[RE_XTM_Y]Z (2),

where Z represents a third element of atomic number 5, 6, 7, 14, 15 or 16 (i.e., B, C, N, Si, P or S) and commonly occurring isotopes of the same.

In formulas (1) and (2), X represents 1 to 5, and Y represents 2 to 29, the balance usually being impurities unavoidably incorporated during production. These impurities are residual from the solvents, reducing agents, and surfactants used in chemical processing. In most cases, these additional elements could be deleterious to the performance figures of merit of the magnets. However, in some cases such additional elements can be incorporated intentionally and can act to modify or stabilize the resultant alloy and positively affect the micro- or nanostructure of the product in such a way as to result in improved figures of merit. One such example would be the use of boric acids as a reducing agent to intentional introduce boron ions to metallic NdFe alloy particles in order to stabilize the highly desired Nd₂Fe_I4B permanent magnet alloy. Exemplary salts to introduce the boron ions include H₃BO₃, B₂O₃, H₄B₄O₇, NaBH₄, triphenyl BNa(OH), and NaCaB₅O₅(OH)₅. Any of these may be hydrated to different degrees. In the use of such boric acid-based reducing agents, the concentration will need to be adjusted to increase the concentration of boron ions in the reaction vessel so as to increase the likelihood of incorporating the boron ions into the metal RE-TM alloy product. The intention is to introduce sufficient boron ions into the growing particle so as to stabilize the RE-TM-B alloys. For example, in Nd₂Fe_I4B, the atomic percentage of B is -5.9%.

Additionally, other elements may be used purposefully in the chemical processing method of the invention to facilitate the co-reduction of the RE- and TM- salts and, thus, realize such [RE_xTMγ] or [RE_xTMy] Z metallic alloys. Such elements may include Au, Pd, Pt, Ir, Zr, Ag, Cu, Bi, Sb, and Pb. As a result, these elements may appear in the final metal alloy particles.

In an exemplary embodiment, metal salts containing the RE and TM components and, if required the Z component, are dissolved in a solvent having the properties of both a reducing agent and an oxidation preventing agent, and the mixture is maintained at ambient temperature or heated up to a temperature of no more than the boiling point of the respective solvent. Exemplary such solvents are the polyols (e.g., a polyalcohol or a derivative of a polyalcohol) . The metal salts are reduced to metal particles and/or alloys with ferromagnetic properties.

The shape of the resulting particles can be, e.g., spherical, truncated cubes, rods, wires, platelets and/or blades, depending on the conditions of reaction. The solvent can be, e.g., one or a combination of the following: ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, tetraethylene glycol, 1,2 propanediol and pentaethylene glycol or its derivatives.

A dispersant (or surfactant) can be incorporated in the reaction solution during the synthesis reaction. The dispersant adheres to the particle surfaces and helps to suppress aggregation among the particles. In addition, the grain diameter of the synthesized particles can be controlled by suitably selecting the type and concentration of the added dispersant. Dispersants suitable for use are ones that easily adhere to the metal particle surfaces. These include surface active agents including a radical, specifically an amine, amide or azo radical, or an organic molecule including either a thiol radical or a carboxyl radical in its structure.

To achieve a rapid reduction of the starting salts to RE- TM alloy particles, one or both of NaOH or KOH can be added, at various ratios, as supplemental oxidation prevention agents. The experiments have been carried out at the laboratory level in N₂ and Ar gas atmospheres with various flow rates, but typically at -500 mL/min. Commercial scale production could employ a different inert gas or a different flow rate. Since the precursor chemistry for the described synthesis process is readily available and cost-effective, there is no anticipated physical or chemical limitation that would limit the scale-up to industrial levels.

A summary of the various process parameters and some of the effects of changing these parameters is given in Table 1.

Table 1 - Variation of Process Parameters and Effects Thereof

In general, the morphology and size of the SmCo nanoparticles were found to strongly depend upon the reaction conditions, such as reaction temperature, metal ion concentration, and molar ratio between the repeating units of PVP. For example, if the molar ratio was changed between the SmCo and PVP, spherical particles of -20 nm were produced. When the reaction temperature was changed, a subtle change in the morphology was recorded. The elongated, rod-shaped nanoparticles were observed at a reaction temperature of 473 K. The key to the formation of uniform nanorods at 473 K is believed to be the use of PVP as a polymer capping reagent. Increasing the reaction temperature to 573 K resulted in the growth of more anisotropic particles, with a high aspect ratio. Once the rod-shaped structure had been formed at a lower temperature, it was readily grown into longer and wider nanoblades at a higher reaction temperature. In the early stages of the reaction process, i.e., at 200 ^°C, the majority of the larger SmCo particles could be directed to grow into nanorods with uniform diameters, which then grew continuously into uniform nanoblades, up to 120 nm in length and about 10 nm in width at 300 ^°C. The nanorods and nanoblades that are described here appear to be shapes that have not yet been demonstrated for SmCo particles prepared through a liquid-phase process, and by the polyol process in particular.

The method of the invention provides for the production of RE-TM metallic particles/powders or films/coatings in a chemical process that does not require additional process steps beyond the chemical synthesis steps, such as an additional heat treatment, to achieve a product having permanent magnet properties. The resulting particles/powders, once rinsed and dried, are typically compacted and sintered to produce a final "engineered" product. The advantage of forming acicular particles, e.g., "blades", rods, or ellipsoids, for the final engineered product is that these shapes respond sensitively to the application of a magnetic field, thus making them easy to align in a high density compact. Such anisotropic products are highly sought for many applications.

It has been suggested in the art that atoms residing on different crystallographic facets may have different interaction strengths with a polymeric capping reagent, thus leading to anisotropic growth of the precipitant. However, identification of the growth mechanism remains elusive. For example, it remains unclear how the SmCo nanorods evolve from nanoparticles at the initial stage of the reaction. It is noteworthy that the final morphology of SmCo nanostructures synthesized using the polyol process of the invention was dependent on the concentration of PVP added to the reaction solution. When the concentration of PVP was relatively high, only SmCo nanoparticles with quasi- spherical shapes were obtained, as shown in Fig. 3 (a) . The absence of particles having the nanoblade morphology in that final product may occur because the selectivity in interaction between the PVP and various crystallographic planes was lost, and thus isotropic growth resulted. Also, the formation of monomers of different structures while using the different types of metal salts also favors the presence of different shaped particles with various types of surfactants.

The following examples are presented to illustrate the advantages of the present invention and to assist one of ordinary skill in making and using the same. These examples are not intended in any way otherwise to limit the scope of the disclosure .

Exemplary Synthetic Process

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol

(boiling point: 587 K) in an amount equivalent to a 1:5 wt . % ratio. The total concentration of metal salts was 0.01 M. PoIy-

(N-vinylpyrrolidone) (PVP), at 0.54 g, was added as a capping agent to further protect the particles from oxidation as well as to control the direction of the particle growth. NaOH, in an amount of 0.5-0.6 g, was added to accelerate the reaction kinetics of the system.

The solution was transferred to a reaction vessel equipped with a condenser, a mantle heater with temperature controller attachment, and an inert gas atmosphere. The reaction vessel was placed on the mantle heater with attached temperature controller, and the solution was heated with stirring at 200 rpm while nitrogen or argon, as an inert gas, was introduced into the reaction vessel at a flow rate of ~ 500 mL/min. The reaction was continued for 2 hrs at a temperature of 563 - 633 K.

After the solution was cooled to room temperature so that precipitation occurred, the reacted solution samples containing precipitated particles were added to methanol or ethanol in sufficient amounts to allow thorough rinsing and were centrifuged in a table top centrifuge at 5000 rpm for 10 minutes. Following removal of the supernatant liquid, the black "powder" precipitate was mixed with 100 mL of methanol (or ethanol) and dispersed using an ultrasonicator . The dispersion was again centrifuged at 5000 rpm for 10 minutes, and the supernatant liquid was removed. The magnetic precipitate obtained was thereafter subjected to four or more additional cycles of rinsing, dispersion in the ultrasonicator, and repeated centrifugation. The substance containing Sm-Co particle powder obtained after final removal of supernatant liquid was dried and subjected, e.g., to X-ray diffraction, vibrating sample magnetometry, electron microscopy and other characterization steps as needed to determine the resulting properties of the powder.

Table 2 presents examples of synthesis conditions, phase presence and magnetic properties of Sm-Co compounds directly synthesized by the method of the invention without annealing. Following the table, a description is given of each example, and the particles obtained are analyzed for their structural and magnetic properties.

Table 2: Examples of Processing Schemes

Effect of molar concentration, Example 1: 0.005M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 rnL of tetraethylene glycol (boiling point: 587 K) in an amount of 1:5 wt.% ratio and 0.005 M salt concentration. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. X-ray diffraction (XRD) patterns (as exemplified in Fig. 1) show the presence of Sm₂COi₇ and SmCos phases. Transmission electron microscopy (TEM) (as exemplified in Fig. 2) shows the presence of blade-like particles with an average length of 100 nm and width of 10 nm. Such a particle morphology is referred to from hereon as "nanoblades" in which the length and width will be defined. The thickness of the blade in most cases could not readily be confirmed. The same type of nanoblades was formed in nearly all examples. As exemplified by the hysteresis loop curves of Fig. 3, a coercivity of about 1 kOe was achieved at room temperature (RT) for the 0.005 M concentration of the Sm and Co acetate metal salt solutions. The elemental spatial distribution of Sm and Co ions were confirmed to be uniformily distributed throughout the particles by elemental mapping analysis within a high resolution transmission microscope.

Effect of molar concentration, Example 2: 0.01M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and 0.01 M salt concentration. The remaining steps are the same as those presented in the exemplary process.

XRD patterns show the presence of Sm-Co alloy phases. The phases consist of Sm₂COi₇ and SmCos structures. TEM microscopy shows the Sm-Co alloy particles having platelet/blade nanostructures. The room temperature coercivity was about 2.7 kOe.

Effect of molar concentration, Example 3: 0.015M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio. The molar salt concentration is 0.015 M. The remaining steps are the same as those presented in the exemplary process.

XRD patterns show the presence of Sm-Co alloy with the main phases being Sm₂COi₇ and SmCos. TEM microscopy shows particles having a platelet-like structure where the length of the platelet was reduced compared with the results of the 0.01 M reaction. The room temperature coercivity was about 3.2 kOe. The hysteresis loop data were measured at 300 K and also at 100 K. At 300 K, the coercivity was about 3.2 kOe whereas when the temperature is decreased to 100 K, the coercivity increased to ~ 8 kOe.

Effect of molar concentration, Example 4: 0.02M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a 0.02 M salt concentration. The remaining steps are the same as those presented in the exemplary process. XRD patterns show the presence of Sm₂COi_? and metallic Co. TEM microscopy shows spheric particles having a platelet structure wherein the length of the platelet was reduced compared with the results of Example 2. The coercivity was determined to be about 3.3 kOe at RT. Each sphere is about 100 nm in diameter and consists of a group of Sm-Co alloy blades with shorter length than other particles resulting from the molar concentrations shown in Example 1.

Effect of reaction temperature, Example 5: 473 K

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a 0.01 M salt concentration. The reaction was carried out at 473 K. The remaining steps are the same as those presented in the exemplary process.

The presence of nearly monodispersed particle nanoblades was shown by x-ray diffraction, the nanoblades having an average length of 100 to 300 nm and width of 5-10 nm, even at 473 K. However, from the poor magnetic properties that may be inferred from the hysteresis loop plots, complete crystallinity was not achieved at this temperature. When the reaction temperature was increased (see subsequent examples), dense and thick SmCo metallic nanoblades were formed, similar to those produced in Examples 1-4. Hence, Examples 5-7 show evidence that the Sm-Co particles having the rod/blade-like morphology are stabilized at lower temperatures with more complete crystallization occuring at higher temperatures.

Effect of reaction temperature, Example 6: 523 K

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The reaction was carried out at 523 K for 2 hrs. The remaining steps are the same as those presented in the exemplary process.

At this temperature the presence of both fee Co and Sm-Co alloy was confirmed as evidenced from the XRD pattern where some peaks were indexed to metallic fee phase that is deduced to be cobalt.

Effect of reaction temperature, Example 7: 578 K

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The reaction was carried at 578 K. The remaining steps are the same as those presented in the exemplary process.

XRD patterns show the presence of Sm-Co phases in the alloy particles, with the Sm₂COi₇ and SmCos phases most prominent. At a reaction temperature of 578 K, nanoblades were formed and aggregated with aggregates having a diameter of about 200 nm. The aggregates form due to magnetic interactions during synthesis. The coercivity at RT is about 3 kOe. Effect of reaction time and molar concentration, Example 8: 4 hrs, 0.005 M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 rnL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.005 M. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 4 hrs in tetraethylene glycol. XRD patterns show the presence of Sm₂COi₇ and SmCos phases and the TEM micrographs show the presence of nanoblade particles having an average length of 100-150 nm and width of 5-10 nm. The coercivity was measured to be about 2.2 kOe at RT.

Effect of reaction time and molar concentration, Example 9: 4 hrs, 0.01 M

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 4 hrs in tetraethylene glycol. The XRD pattern shows the presence of Sm₂COi₇ and SmCos phases. The TEM micrographs show nanoblades with irregular surfaces. The coercivity was measured to be about 3 kOe at RT.

Effect of reaction time and molar concentration, Example 10: 1 hr, 0.01 M

Sm (HI) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 60 minutes in tetraethylene glycol. XRD patterns show the presence of SmCo₅ and Sm₂COi₇ alloy phases. The coercivity was measured to be about 3.4 kOe at 300 K.

Effect of addition of Fe (nanocomposite magnets), Example 11: 2.5 wt.% Fe

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The concentration of Fe acetate was 2.5 wt.%. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns in Fig. 11 (a) show the presence of Sm₂COi₇ and SmCo₅ phases and a metallic bcc Fe phase was deduced. The coercivity value at room temperature was -2.5 kOe.

Effect of addition of Fe (nanocomposite magnets), Example 12: 10 wt.% Fe

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The concentration of Fe- acetate was 10 wt.%. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of Sm₂COn and bcc Fe phases. The TEM micrograph shows aggregates of nanoblades. A coercivity of about 3.2 kOe was measured at 300 K.

Effect of addition of Fe (nanocomposite magnets), Example 13: 20 wt.% Fe

Sm (HI) acetate and Co(II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:5 wt.% ratio and a salt concentration of 0.01 M. The concentration of Fe-chloride (i.e. FeCl₂.4H₂O) was 20 wt.%. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of Sm₂COn and metallic Fe phases. The TEM micrograph shows aggregates of nanoblades. The coercivity for this sample was measured at room temperature to be 2.7 kOe.

Effect of Sm/Co ratio, Example 14: 1:4 wt.% ratio

Sm (HI) acetate and Co(II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 1:4 wt.% ratio and a salt concentration of 0.01 M. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of the Sm₂COi₇ phase. The TEM micrographs show aggregates of uniform length consisting of nanoblades of about 50 nm in length and 20 nm in width. A coercivity of about 2.6 kOe was measured at room temperature.

Effect of Sm/Co ratio, Example 15: 2:7 wt.% ratio

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in an amount of 2:7 wt.% ratio and 0.01 M salt concentration. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried out at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of SmCo₅ and Sm₂COi₇ alloy phases. A coercivity of about 2.6 kOe was measured at 300 K.

Effect of Sm/Co molar cone, ratio (Examples 16-18)

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol in different molar concentration ratios of Sm: Co between 10:90 and 50:50. The remaining steps are the same as those presented in the exemplary process. Exemplary results are described in Examples 16-18). The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of the Sm₂COi₇ and SmCos phases. TEM shows nanoblades of length about 200 nm and width of 40 nm. A coercivity in the range of between 1 and 9.2 kOe and magnetization between 40 and 80 emu/g were measured at RT.

Effect of Sm/Co molar cone, ratio, Example 16: 30:70

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylelene glycol in an amount of 30:70 molar cone, ratio. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of the Sm₂COi₇ phase. TEM shows nanoblades of length about 200 nm and width of 40 nm. A coercivity of about 2.8 kOe was measured at 300 K.

Effect of Sm/Co molar cone, ratio, Example 17: 20:80

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylelene glycol in an amount of 20:80 molar cone, ratio. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of SmiCo₂, SmiCos, Sm₂COi₇ phases. A coercivity of about 3.0 kOe was measured at RT.

Effect of Sm/Co molar cone, ratio, Example 18: 10:90

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylelene glycol in an amount of 10:90 molar cone, ratio. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of SmiCo₂, SmiCos, Sm₂COi₇ phases and a mixture of fee and hep-Co. A coercivity of about 1.0 kOe was measured at 300K.

Effect of PVP, Example 19: 0.06 g

Sm (III) acetate and Co (II) acetate tetrahydrate were added to and dissolved in 100 mL of tetraethylene glycol as discussed in Example 1. The weight of PVP (polyvinyl pyrolidone) is 0.06 g instead of 0.54 g used in all the above experiments. The remaining steps are the same as those presented in the exemplary process.

The reaction was carried at 568 K for 2 hrs in tetraethylene glycol. XRD patterns show the presence of the Sm₂COi₇ phase. A coercivity of about 2.5 kOe was measured at 300K. References :

Gu, H. W., B. Xu, J. C. Rao, R. K. Zheng, X. X. Zhang, K. K.

Fung, C. Y. C. Wong, J. Appl . Phys . 2003, vol. 93, 7589. Hou, Y., S. Sun, C. Rong and J. P. Liu, Appl. Phys. Letts, 2007, vol. 91, 153117. Hou, Y., Z. Xu, S. Peng, C. Rong, J. P. Liu, and S. Sun,

Advanced Materials, 2007, Vol. 19, 3349. Matushita, T., J. Masuda, T. Iwamoto and N. Toshima, Chemistry

Letters, 2007, Vol. 36, 1264. Shin, S. J., Y. H. Kim, H. G. Cha, C. W. Kim and Y. S. Kang,

MoI. Cryst. Liq. Cryst . 2007, Vol. 464, pp. 39/621-49/631.

While the present invention has been described in conjunction with a preferred embodiment, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof.

Claims

CLAIMS What is claimed is:

1. A method of making a rare earth-transition metal magnetic material, said method comprising the steps of: combining a rare earth metal salt and a transition metal salt in a solvent to form a mixture, wherein said solvent is a pure solvent or a solvent solution and wherein said pure solvent or said solvent solution has the properties of both a reducing agent and an oxidation prevention agent; causing precipitation of particles of a magnetic material; and collecting precipitating said particles from said mixture.

2. The method of claim 1, wherein said precipitating particles are nanoparticles .

3. The method of claim 1, wherein said precipitating particles are microparticles .

4. The method of claim 1, wherein said causing and said collecting steps are carried out simultaneously and wherein said particles are precipitated in the form of a film or a coating on a substrate.

5. The method of claim 1, wherein said counter anion in said rare earth (RE) metal salt is selected from the group consisting of RE-chlorides, RE-sulphates, RE-acetylacetonates, RE-acetates, RE-hydroxides, RE-nitrates and RE-flourides .

6. The method of claim 1, wherein said counter anion in said transition metal (TM) salt is selected from the group consisting of TM-chlorides, TM-sulphates, TM-acetyl acetonates, TM- acetates, TM-hydroxides, TM-nitrates and TM-flourides .

7. The method of claim 1, wherein said rare earth metal is selected from the group consisting of Sm, La, Ce, Pr, Nd, Y, Gd, Tb, Dy, Ho, Pm and Er.

8. The method of claim 1, wherein said transition metal is selected from the group consisting of Fe, Co, Ni, Cr and Mn.

9. The method of claim 1, wherein said solvent is a polyol or a glycol.

10. The method of claim 1, wherein said solvent or solvent solution is supplemented with a hydroxide or a boric acid derivative .

11. The method of claim 10, wherein said hydroxide is NaOH or KOH.

12. The method of claim 10, wherein said boric acid derivative is selected from the group consisting of H₃BO₃, B₂O₃, H₄B₄O₇, NaBH₄, triphenyl BNa(OH) and NaCaB₅O₅(OH)₅.

13. The method of claim 1, wherein a salt of an element selected from the group consisting of B, C, N, Si, P and S is also included in said combining step with said rare earth metal salt and said transition metal salt.

14. The method of claim 1, wherein an element selected from the group consisting of Au, Pt, Ir, Zr, Ag, Cu, Bi, Sb, Pb, Pd and B.

15. The method of claim 1, including the step of adding a surfactant to stabilize particle precipitation.

16. The method of claim 1, wherein said causing step comprises introducing energy into said mixture.

17. The method of claim 1, further comprising the step of processing said particles into a final product.