Coercivity-enhanced iron nitride nanoparticles with high saturation magnetization

Abstract

To provide coercivity-enhanced iron nitride nanoparticles with high saturation magnetization.SOLUTION: Iron nitride nanoparticles and magnetic materials formed from iron nitride nanoparticles are described. The iron nitride nanoparticles have a core and shell morphology. The shell provides means to nitride the core. The magnetic materials have an Msat greater than about 160 emu / g and a coercivity greater than about 700 Oe.SELECTED DRAWING: Figure 1-2

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Patent Application No. 62 / 851,190, filed May 22, 2019, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The disclosed invention is in the field of iron nitride and magnetic materials made therefrom. [Background technology]

[0003] Iron nitride nanoparticles have been produced for many years by the controlled nitridation of iron and iron oxide nanoparticle precursors. These materials are of interest due to their potential for processing into high-performance non-rare earth-containing permanent magnets. α''-Fe, which has a high coercivity (Hc>1000 Oe), 16 Iron nitride nanoparticles with the N2 phase have been found to have low saturation magnetization (MSat<180 emu / g), and thus, to date, commercially viable iron nitride permanent magnets have not yet been developed. Therefore, to produce permanent magnets characterized by high coercivity (Hci>1000 Oe) and high saturation magnetization (MSat>180 emu / g), α''-Fe 16 There is a need to produce iron nitride nanoparticles having the N2 phase. Summary of the Invention

[0004] One problem addressed by the present invention is that iron nitride nanoparticles with high coercivity (H > 1000 Oe) are found to have low saturation magnetization (MSat < 180 emu / g). Microscopic examination of the nanoparticles reveals that they are characterized by a core / shell structure with a non-ferromagnetic iron oxide shell. While not bound by any theory of operation, the shell is believed to enhance the coercivity by magnetically separating adjacent nanoparticles. However, the shell thickness has been found to be typically at least 5 nm. This thickness results in a significant weakening of the overall saturation magnetization of the core / shell nanoparticles. This weakening of saturation magnetization substantially limits the performance of permanent magnets fabricated from the iron nitride nanoparticles.

[0005] Another problem solved by the present invention is that certain potentially advantageous nanoparticle shell materials, e.g., thin conformal materials of uniform thickness, are typically α''-Fe 16 The main problem is that the temperature required for the N2 phase is about 200°C higher than the maximum stable temperature. 16 For structures consisting of a continuous shell around N2, these high-temperature materials are α''-Fe 16 After the N2 phase is formed in the core of the nanoparticles, it cannot be provided by known deposition processes, and if provided by known deposition processes, it is α''-Fe 16 The N2 phase decomposes. Low temperature deposition methods exist, such as electrolytic and electroless deposition, but they are very difficult to control and do not easily deposit thin materials on nanoparticles. Examples of materials that require temperatures above 200°C for the formation of a shell layer by known deposition methods include nitrides, such as AlN, or ductile metals, such as Cu, Al, Sn, and Zn. These materials, along with α''-Fe 16 To allow for the combination of N2 phase with α''-Fe, a novel structure is provided that offers additional advantages during the formation of bulk magnets. 16Maximizing the saturation magnetization of compressed bulk magnets composed of N2 core-shell nanoparticles to produce commercially acceptable α''-Fe 16 To help prepare N2-based bulk magnets.

[0006] The present invention solves this problem by: 1) limiting the shell thickness to 5 nm or less, preferably 4 nm or less, preferably 3.5 nm or less, preferably 3 nm or less, preferably 2.5 nm or less, even more preferably 2 nm or less, and even more preferably 1 nm or less; and 2) requiring a minimum core diameter of at least 20 nm, preferably at least 30 nm, preferably at least 40 nm, and even more preferably at least 50 nm (core diameters are typically less than 100 nm or 200 nm). As the core diameter increases and the shell thickness decreases, the overall saturation magnetization of the core / shell magnet increases. An advantage of the present invention is that it produces iron nitride nanoparticles with limited weakening of saturation magnetization, having a saturation magnetization of at least 180 emu / g, preferably at least 195 emu / g.

[0007] Therefore, the present invention provides a composition comprising 50% or more by weight of α''-Fe 16 The present invention provides a plurality of iron nitride nanoparticles comprising an iron-based core comprising an N2 phase and a thin material deposited adjacent to the iron-based core, wherein the thin material is configured to enable nitridation of the iron-based core. Preferably, the iron nitride nanoparticles are monocrystalline α″-Fe with a non-magnetic shell. 16The thin material contains a N2 core. This is because nanoparticles with multiple randomly oriented crystal grains tend to be less magnetically aligned. Typically, suitable thin materials useful for making magnetic materials include non-ferromagnetic materials. Non-limiting examples of suitable non-ferromagnetic thin materials include FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof. Often, the thin material is provided as a continuous thin shell on the core, but the thin material can also be a discontinuous shell, such as a discontinuous shell having irregularities, perforations, holes, or both on the core nanoparticle. The thin material covering the core can also be appropriately configured to be a substantially continuous shell through which nitrogen can diffuse. Exemplary iron nitride nanoparticles are characterized by an MSat greater than about 145 emu / g and a coercivity greater than about 1000 Oe.

[0008] In many embodiments, the nanoparticles are characterized by X-ray diffraction using an X-ray source having a Cu target configured to emit single wavelength Cu-Kα X-rays, which produces a diffraction peak (at about 42.7° 2θ) of α″-Fe 16 The ratio of the integrated intensity of the α-Fe(211) peak (at 2θ of about 82.2°) to the N2(202) peak is 0.2 or less, preferably 0.15 or less, or even 0.10 or less.

[0009] Furthermore, the present invention provides a magnetic material having an α''-Fe content of 50% or more by weight. 16 A magnetic material is provided, comprising a plurality of iron nitride nanophases, characterized by including an N2 phase, and a non-ferromagnetic material disposed between the nanophases. Preferably, the non-ferromagnetic material disposed between the nanophases comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof. Exemplary magnetic materials are characterized by having MSat greater than 160 emu / g and a coercivity greater than about 700 Oe.

[0010] In certain embodiments, the present invention provides core / shell iron nitride nanoparticles having a non-ferromagnetic shell, some of the non-ferromagnetic shell being 1-2 nm thick.

[0011] In another embodiment, the present invention provides iron nitride nanoparticles characterized by having a coercivity greater than about 2000 Oe and a saturation magnetization greater than about 195 emu / g.

[0012] In certain embodiments, the present invention provides a 1-2 nm thick non-ferromagnetic shell on an iron nitride core.

[0013] In another embodiment, the present invention uses low temperature passivation to form a 1-2 nm thick non-ferromagnetic shell on the iron nitride core.

[0014] In some embodiments, the present invention uses amorphous iron oxide nanoparticles as precursors.

[0015] In another embodiment, the present invention provides nanoparticle structures having a predominantly α-Fe core and discontinuous shell layers.

[0016] In some embodiments, the present invention provides a method for producing a predominately α″-Fe 16 Iron nitride nanoparticles are provided, characterized by having a core of N2 and a discontinuous shell layer.

[0017] In certain embodiments, the shell is provided before the nitride phase, and the shell is discontinuous and does not coat the entire area of ​​the core, and is then formed, for example by nitriding, into a layer of primarily α″-Fe 16 The Fe core is nitrided, consisting of α-Fe phase, resulting in a core of N2, which is surrounded by a discontinuous shell.

[0018] In another embodiment, the present invention provides a method for producing a nitridable iron core comprising a predominately α″-Fe alloy with a continuous shell composed of a material that allows for nitridation of the Fe core. 16The present invention provides nanoparticles characterized by a structure having a core of N2, for example, due to the inherent properties of the shell material or because the shell is thin, for example, less than 3 nm thick.

[0019] In certain embodiments, the present invention provides core / shell iron nitride nanoparticles in which a continuous shell is provided prior to the nitridation of the phase, which allows for nitridation of the Fe core despite the presence of the shell, and then, e.g., by nitridation, to form primarily α″-Fe 16 The Fe core is nitrided, consisting of the α-Fe phase, resulting in a core of N2, said core being surrounded by a discontinuous shell.

[0020] In another embodiment, the present invention provides a nanocrystalline silicon nanotube comprising a nanocrystalline silicon nanotube having a first discontinuous shell and a second thin continuous or discontinuous shell layer less than 3 nm thick, comprising predominantly α″-Fe 16 The present invention provides nanoparticles characterized by having a structure with a core of N2.

[0021] In some embodiments, a first shell is provided before the nitride phase to provide core / shell iron nitride nanoparticles in which the shell is discontinuous and does not coat the entire area of ​​the core. This is then converted, for example, by nitridation, to a primarily α″-Fe 16 The Fe core is nitrided, consisting of α-Fe phase, resulting in a core of N2, surrounded by a discontinuous shell. A second shell is then provided, the second shell being primarily composed of α''-Fe. 16 The N2 core and the first shell material are coated either continuously or discontinuously.

[0022] The general description and the following detailed description are exemplary or explanatory only and are not restrictive of the invention, which is defined in the appended claims. Other aspects of the invention will become apparent to those skilled in the art from the detailed description of the invention provided herein.

[0023] The summary, as well as the following detailed description, will be better understood when read in conjunction with the accompanying drawings. For the purpose of illustrating the invention, there are shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods, components, and apparatus disclosed. Additionally, the drawings are not necessarily drawn to scale. [Brief explanation of the drawings]

[0024] [Figure 1-1] 1 provides the hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 1. [Figure 1-2] 1 provides an X-ray diffraction pattern of the aluminum oxide coated iron nitride nanoparticles produced in Example 1. [Figure 1-3] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 1. [Figure 1-4] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 1. [Figure 1-5] 1 shows an elemental map of aluminum localized at the surface of aluminum oxide coated iron nitride nanoparticles according to Example 1. [Figure 1-6] 1 shows an elemental map of iron distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 1. [Figure 1-7] 1 shows an elemental map of nitrogen distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 1. [Figure 1-8] 1 provides images of aluminum oxide coated iron nitride nanoparticles from which elemental maps of Al, Fe, and N were obtained in Example 1. [Figure 1-9] 1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 1 to a pressure of 10 MPa. [Figure 1-10]1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 1 to a pressure of 12 MPa. [Figure 2-1] 1 provides the hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 2. [Figure 2-2] 1 provides an X-ray diffraction pattern of aluminum oxide coated iron nitride nanoparticles produced in Example 2. [Figure 2-3] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 2. [Figure 2-4] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 2. [Figure 2-5] 1 provides an elemental map of aluminum localized at the surface of aluminum oxide coated iron nitride nanoparticles according to Example 2. [Figure 2-6] 1 provides an elemental map of iron distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 2. [Figure 2-7] 1 provides an elemental map of nitrogen distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 2. [Figure 2-8] 1 provides a TEM image of aluminum oxide coated iron nitride nanoparticles from which elemental maps of Al, Fe, and N were obtained in Example 2. [Figure 2-9] 1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 2 to a pressure of 10 MPa. [Figure 2-10] 1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 2 to a pressure of 12 MPa. [Figure 3-1]1 is a hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 3. [Figure 3-2] 1 provides an X-ray diffraction pattern of aluminum oxide coated iron nitride nanoparticles produced in Example 3. [Figure 3-3] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 3. [Figure 3-4] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 3. [Figure 3-5] 1 provides an elemental map of aluminum localized at the surface of aluminum oxide coated iron nitride nanoparticles according to Example 3. [Figure 3-6] 10 provides an elemental map of iron distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 3. [Figure 3-7] 10 provides an elemental map of nitrogen distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 3. [Figure 3-8] 1 provides a transmission electron microscope image of aluminum oxide coated iron nitride nanoparticles from which elemental maps of Al, Fe, and N were obtained in Example 3. [Figure 4-1] 1 provides the hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 4. [Figure 4-2] 1 provides the hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 4. [Figure 4-3] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 4. [Figure 4-4] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 4. [Figure 4-5]1 provides an elemental map of aluminum localized at the surface of aluminum oxide coated iron nitride nanoparticles according to Example 4. [Figure 4-6] 10 provides an elemental map of iron distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 4. [Figure 4-7] 10 provides an elemental map of nitrogen distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 4. [Figure 4-8] 1 provides a transmission electron microscope image of aluminum oxide coated iron nitride nanoparticles from which elemental maps of Al, Fe, and N were obtained in Example 4. [Figure 5-1] 1 provides the hysteresis loop of aluminum oxide coated iron nitride nanoparticles produced in Example 5. [Figure 5-2] 1 provides an X-ray diffraction pattern of aluminum oxide coated iron nitride nanoparticles produced in Example 5. [Figure 5-3] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 5. [Figure 5-4] 1 provides a transmission electron microscope image measuring the thickness of the aluminum oxide coating on the iron nitride nanoparticles produced in Example 5. [Figure 5-5] 10 provides an elemental map of aluminum localized at the surface of aluminum oxide coated iron nitride nanoparticles according to Example 5. [Figure 5-6] 10 provides an elemental map of iron distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 5. [Figure 5-7] 10 provides an elemental map of nitrogen distributed throughout the mass of aluminum oxide coated iron nitride nanoparticles according to Example 5. [Figure 5-8] 1 provides a TEM image of aluminum oxide coated iron nitride nanoparticles from which elemental maps of Al, Fe, and N were obtained in Example 5. [Figure 5-9]1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 5 to a pressure of 10 MPa. [Figure 5-10] 1 provides the hysteresis loop of an isotropic compressed magnet made by uniaxially compressing the aluminum oxide coated iron nitride nanoparticles produced in Example 5 to a pressure of 12 MPa. [Figure 6] The closest packing configuration is shown for a random packing of 10,000 polydisperse spheres at density φ = 0.662. The distribution of sphere radii is lognormal with a standard deviation of σ = 0.3. [Courtesy of Baranau and Tallarek, "Random-close packing limits for monodisperse and polydisperse hard spheres," Soft Matter., Vol. 10, 2014, pp. 3826-41] [Figure 7-A] 1 shows a contour plot of the saturation magnetization (Bsat) of a bulk iron nitride magnet comprising bonded iron nitride nanoparticles made according to the present invention, illustrating how the saturation magnetization (Bsat) varies with the nanoparticle core diameter d (nm) and thickness t (nm) for a nanoparticle core Bsat of 225 emu / gram and a volume filling factor of 63%. [Figure 7-B] 1 shows a contour plot of the saturation magnetization (Bsat) of a bulk iron nitride magnet comprising bonded iron nitride nanoparticles made according to the present invention, illustrating how the saturation magnetization (Bsat) varies with the nanoparticle core diameter d (nm) and thickness t (nm) for a nanoparticle core Bsat of 250 emu / gram and a volume filling factor of 63%. [Figure 8] 1 is an alignment of the characteristics of two exemplary iron nitride core / shell nanoparticles (Case 1 and Case 2) made according to the present invention. [Figure 9] 1 is a list of features of an exemplary magnet composed of iron nitride core / shell nanoparticles made according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention may be better understood by reference to the following detailed description in conjunction with the accompanying drawings and examples, which form a part of this disclosure. It is understood that the present invention is not limited to the specific devices, methods, applications, conditions, or parameters described and / or illustrated herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to limit the claimed invention. Furthermore, unless the context clearly dictates otherwise, as used in this specification, including the appended claims, the singular forms "a," "an," and "the" include plurals, and reference to a particular numerical value includes at least that particular value. As used herein, the term "plural" means more than one. When ranges of values ​​are expressed, certain other embodiments include from one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximate values ​​by use of the prefix "about," it is understood that the particular value forms certain other embodiments. All ranges are inclusive and combinable.

[0026] Certain features of the invention that are, for clarity, described herein in the context of individual embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination of the individual. Furthermore, references to values ​​recited in ranges include each and every value within that range.

[0027] The present invention allows the formation of core / shell iron nitride nanoparticles, the core of which is composed of at least 50%, preferably at least 80%, mass fraction of α''-Fe 16 The remaining mass fraction of the core may consist of α-Fe, α'-FeN, γ'-FeN, and ε-Fe 2-3 It may consist of a mixture of N phases.

[0028] The thin material for the shell may be composed of one or more non-ferromagnetic materials or phases in the shell. Non-limiting examples of non-ferromagnetic materials include FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, AlN, MnN, Cu, Al, Sn, and Zn.

[0029] The core diameter may be 20 to 200 nm, preferably 20 to 100 nm, and even more preferably 40 to 80 nm. The shell thickness may be 0.1 to 4.0 nm, preferably 0.1 to 1.0 nm. The shell material may be less than 4 nm, less than 3.5 nm, less than 3 nm, less than 2.5 nm, less than 2 nm, or less than 1.5 nm.

[0030] The shape of the core / shell nanoparticles can be spherical or non-spherical. Examples of non-spherical shapes include rods, needles, prolate spheroids, oblate spheroids, disks, and plates. Further examples of non-spherical shapes include porous structures such as foams, gels, and sponges.

[0031] The core can be produced by a method including nitriding a nanoparticle precursor. The nanoparticle precursor may include α-Fe, Fe3O4, α-Fe2O3, and γ-Fe2O3. The nanoparticle precursor may include amorphous nanoparticles composed of iron, iron oxide, iron nitride, iron boride, and mixtures thereof. When an iron oxide nanoparticle precursor is used, the nanoparticles may undergo a reduction treatment before nitriding. When an α-Fe nanoparticle precursor is used, the nanoparticles may be directly nitrided. When the α-Fe nanoparticle precursor has a pre-existing oxide shell, the α-Fe nanoparticle precursor may undergo a reduction treatment before nitriding. The α-Fe nanoparticle precursor may undergo an oxidation treatment before the reduction and nitriding treatment.

[0032] The non-ferromagnetic shell can be formed on the core using any type of suitable chemical deposition technique. Another way to form the shell is to expose the iron nitride core to an atmosphere with a controlled oxidizing potential. Yet another way to form the shell is to precipitate the shell on the surface by chemical and / or electrochemical reaction.

[0033] Atomic layer deposition (ALD) is a versatile technique for producing thin, conformal coatings on a wide variety of surfaces and substrates. Originally, ALD was developed for the fabrication of semiconductor devices as an alternative to other methods, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD). The principle of operation is the sequential adsorption of reactant compounds onto the exposed substrate surface. The adsorption reaction is self-limiting and stops when a uniform surface layer completely coats the substrate. The introduction of the next reactant in the sequence 1) reacts with the first reactant layer to form a uniform layer of the first atoms in the compound being coated, thus forming a new substrate surface, and 2) adsorbs a new layer composed of the next reactant layer. In this manner, thin films with precisely controlled composition and thickness can be grown one atomic layer at a time. A wide variety of exemplary ALD reactants for forming a wide variety of compositions suitable for use as thin materials disposed adjacent to iron nitride nanoparticle cores are provided in the table below.

[0034] Table of ALD reactants suitable for creating thin materials located adjacent to iron nitride nanoparticle cores [Table 1-1] [Table 1-2] [Table 1-3] [Table 1-4] [Table 1-5] [Table 1-6] [Table 1-7] [Table 1-8] [Table 1-9] [Table 1-10]

[0035] Individual powders and particles can also be used as substrates in ALD coating processes. The use of a fluidized-bed reactor allows for intimate mixing of the powder and vaporized ALD reactants. Mechanical agitation, such as a vibrator attached to the reactor, can facilitate particle fluidization. In this configuration, the fluidized-bed reactor is filled with a batch of powder fluidized in a column with vertical gas flow. The ALD reactants are introduced sequentially into the reactor, and a purge step is used to remove excess reactants when changing from one ALD reactant to the next in the sequence. The reactor can be contained within a furnace capable of heating the fluidized bed to the reaction temperature required for ALD.

[0036] A fluidized bed reactor can be used to coat the surface of magnetic iron nanoparticles with an aluminum oxide coating. The purpose of the aluminum oxide coating is to passivate the surface of the iron nanoparticles and inhibit oxidation of the nanoparticles when exposed to air. Suitable ALD reactants include trimethylaluminum (TMA) and water (HO). In the aluminum oxide coating, TMA is the aluminum source and HO is the oxygen source. A suitable ALD reaction temperature can be at least 180°C, and multiple cycles of the TMA + HO ALD process can be used to form an AlO shell on the iron nanoparticle core. Ideally, the thickness of the aluminum oxide shell is less than 5 nm.

[0037] Iron oxide nanoparticles can also be converted to iron nitride nanoparticles using a fluidized-bed reactor. The fluidized-bed reactor may be configured to flow both H2 and NH3 gases through a fluidized bed of nanoparticles at elevated temperatures. A suitable reactor can first reduce the iron oxide nanoparticles to iron using a flow of H2 at a temperature of 350°C to 500°C. Then, in the same reactor, the iron nanoparticles can be transformed into iron nitride nanoparticles using a flow of NH3 at a temperature of 110°C to 180°C. The iron nitride nanoparticles can then be cooled to room temperature and passivated by briefly flowing a mixture of N2 and air through the nanoparticles. The passivation process results in a layer of iron oxide on the surface of the nanoparticles that inhibits further oxidation. A suitable process can produce iron nitride nanoparticles with a saturation magnetization greater than 200 emu / g and a coercivity as high as 1000 Oe.

[0038] Forming a thin shell of material by exposing the iron nitride core to an atmosphere with a controlled oxidation potential can be accomplished by flowing a gaseous mixture of oxygen and an inert gas over the iron nitride core. The exposure can occur immediately after nitriding. The inert gas used in the mixture can include helium, argon, nitrogen, or mixtures thereof. The volume fraction of oxygen in the mixture can be 20% or less, preferably 5% or less. The thickness of the shell can be limited by using high gas flow rates and low gas temperatures to remove the heat of reaction.

[0039] The nitriding and / or shelling can be carried out in a stationary or rotating reactor, which may be a tube, double cone, or another shape that facilitates the reaction. The nitriding and / or shelling can also be carried out in a fluidized bed reactor.

[0040] The reaction vessel may be vibrated intermittently or continuously during the nitriding and / or shelling process. Vibration may improve mixing during the process.

[0041] A limitation of nanoparticle processing is their tendency to form agglomerates. Generally, nanoparticle agglomerates reduce yield by preventing surface-mediated reactions from going to completion. In the case of permanent magnets, agglomerates reduce performance by preventing the production of magnetically anisotropic magnets.

[0042] Certain processes in nitriding and / or shell formation can benefit from control of agglomerate size, size distribution and / or bulk density. For example, operation of a fluidized bed reactor may only be practical if the agglomerates are in a size and bulk density range that allows for fluidization.

[0043] Methods for deagglomerating and / or controlling the agglomeration of nanoparticles include shear mixing, low-energy milling, and ultrasonic treatment, which can be used before, during, or after nitriding and / or shell formation.

[0044] Furthermore, it is recognized that the properties of the final permanent magnet may depend on the size distribution of the nanoparticle precursors, and for this reason it may be necessary to modify the size distribution of the nanoparticles before, during or after the nitriding and / or shelling steps.

[0045] Methods for controlling the size distribution of nanoparticles include filtration, sieving, and mechanical milling. Filtration methods include magnetic separation, electrostatic separation, and centrifugation in a cyclone. Sieving methods include the use of screens and selectively and non-selectively permeable membranes. Mechanical milling methods include sonication, high-energy ball milling, and jet milling.

[0046] In one other embodiment, the present invention specifically provides a method for producing a crystalline ... 16 The present invention relates to a structure comprising nanoparticles having a partial, incomplete, or discontinuous shell surrounding a core of N2, and an associated method for making the structure. Hereinafter, this type of nanoparticle shell layer is defined as a discontinuous shell. A discontinuous shell means that there is a portion of the surface of the core particle that is not covered by the shell material. By having a discontinuous shell, the nitridation process can occur after the shell is formed. This eliminates limitations on the temperature used in forming the shell, thereby enabling the production of α''-Fe nanoparticles with improved magnetic properties. 16 A novel set of starting materials for producing bulk magnets containing the N2 phase is provided.

[0047] In one other embodiment, the present invention specifically relates to a structure comprising nanoparticles having a substantially continuous shell layer composed of a material that is thin enough to allow nitridation of Fe through the shell layer or to allow nitridation of Fe to proceed despite the presence of the shell layer, and an associated method for making the structure. The shell material, the thickness of the shell material, or both the thickness and the shell material, provide a means for allowing nitridation after the shell material is provided. The shell thickness requirement may depend on the material, but may be approximately 1-3 atomic layers, or 3 nm or less, or thicker. Examples of materials that allow nitridation of Fe despite the presence of the shell material include shell materials with pinholes or porosity, or shell materials that inherently allow nitrogen (N) diffusion through their thin solid shell. The diffusing species may be N atoms, N molecules, NH molecules, or some intermediate form, such as (NH) + ions. Furthermore, diffusion can occur as a gas throughout the volume of the shell, along the surface of the pinholes, or through the pinholes / discontinuities if the pinholes are large enough. More generally, the thin material forming the shell layer is configured such that mass transport of the nitrogen-containing species via mechanisms can include volume and / or surface diffusion. The advantage is similar to that of the discontinuous shell structure, in that the nitridation step can occur after the shell is formed, again removing the temperature limitation imposed by the temperature used in forming the shell.

[0048] In one other embodiment, the present invention provides a method for producing a crystalline ... 16 The structure includes a N2 core, a discontinuous first shell material, and a thin (less than 3 nm) continuous second shell material to create the structure. The second thin continuous shell material allows for processing of the nanopowder with some exposure to an oxygen-containing environment during the manufacturing process, which can simplify the manufacturing process. However, this second shell may have a negative effect on the saturation magnetization of the bulk, fully densified final magnet.

[0049] A further advantage of any of these embodiments is realized during densification and compaction of nanoparticles into a bulk magnet. For example, during bulk magnet formation processes using processes such as high shear deformation, if the shell structure of the nanoparticles is composed of a ductile metal, e.g., Cu, Al, Sn, or Zn, the shell regions of the ductile metal in the individual nanoparticles can be compacted and bonded, resulting in the formation of α-′′Fe in the core region of the nanoparticles in the densified material. 16 A continuous or nearly continuous shell structure is formed around the N2 phase. Without being bound by theory of operation, the continuous or nearly continuous shell formed by subsequent compression is composed of a minimal amount of non-magnetic material, reducing or even eliminating the volume of the bulk magnet composed of binder, thereby maximizing the saturation magnetization of the material. Furthermore, the bound shell region provides magnetic separation, enabling single domain or single domain-like coercivity mechanisms. Alternatively, if less discontinuous shell material is used in the pre-densified nanoparticle structure, a domain wall pinning coercivity mechanism can be exerted following compression, along with a higher saturation magnetization.

[0050] The various embodiments described provide a means to nitride the iron core of nanoparticles after the formation of a shell (continuous or discontinuous), removing limitations on processing temperatures for the formation of shell structures. This allows for the formation of either continuous or discontinuous shell structures composed of one or more materials that were previously unavailable. The methods described herein allow for the use of materials such as certain oxides such as Al2O3, nitrides such as AlN, ductile metals such as Cu, Al, Sn, and Zn, combinations of these materials, or other materials with properties that further enhance the properties of the nanoparticles and / or the compressed bulk magnet material. [Example]

[0051] Example 1 The starting material for Example 1 was iron nitride nanoparticles prepared by reduction and nitridation of commercially sourced γ-FeO nanoparticles in a 2-inch diameter rotary tube furnace. A total of nine lots of nanoparticles, each approximately 1.2 grams in mass, were combined together to form the starting material for the aluminum oxide coating operation. First, the γ-FeO nanoparticles were transformed into α-Fe nanoparticles by reduction in flowing H gas at 200 sccm (standard cubic centimeters per minute) at 335°C for 12 hours. The α-Fe nanoparticles were then transformed into iron nitride nanoparticles by flowing NH gas at a rate of 60 sccm at 135°C for 22 hours. The iron nitride nanoparticles were removed and stored in a nitrogen-filled glove box. Phase analysis by X-ray diffraction revealed that the iron nitride nanoparticles were α''-Fe. 16 N2 and ε-Fe 2-3 The phase fraction was a mixture of α''-Fe and N. 16 It was N2.

[0052] The aluminum oxide coating was deposited by atomic layer deposition (ALD) in a fluidized-bed reactor. The reactor was loaded with 10 grams of iron nitride nanoparticles in a nitrogen (N2)-filled glovebox. The reactor was heated to 100 °C, and the nanoparticle bed was fluidized in a flowing N2 carrier gas. The aluminum oxide coating was deposited one atomic layer at a time by alternately introducing vaporized trimethylaluminum (TMA) and water in the N2 carrier gas for a total of nine cycles. Each cycle began with flowing TMA until the methane signal on the residual gas analyzer weakened and the trimethylaluminum signal increased sharply (indicating that all surfaces in the reactor were coated with aluminum). Before flowing HO, the reactor was purged with flowing N2. HO was continued until the HO signal increased sharply (indicating that all surfaces in the reactor were coated with aluminum oxide). Before starting the next cycle, the reactor was again purged with flowing N2.

[0053] The resulting aluminum oxide coated iron nitride nanoparticles were removed in air and their magnetic properties and microstructure were characterized. Figure 1-1 shows the hysteresis loop of the coated nanoparticles. The intrinsic coercivity was measured to be 2008 Oe and the saturation magnetization was measured to be 191 emu / gram. Figure 1-2 shows that the coated nanoparticles still retained the α''-Fe 16 N2 phase and ε-Fe 2-3 The X-ray diffraction pattern shows a mixture of N and α-Fe phases. The major X-ray diffraction peaks of each phase present in Example 1 are identified below in Table 1. Quantitative phase analysis by Mössbauer spectroscopy revealed that the α-Fe 16 This indicates that the mass fraction of the N2 phase is 74%. Table 1: X-ray diffraction peaks of each phase present in Example 1 [Table 2]

[0054] High-resolution transmission electron microscopy was performed to measure the thickness of the aluminum oxide coating on the surface of the iron nitride nanoparticles. Figures 1-3 and 1-4 show the presence of a uniform coating on the nanoparticles, with a thickness of 1.6 to 2.5 nm. Elemental maps were obtained using energy dispersive X-ray analysis. Figure 1-5 shows an elemental map showing that the aluminum signal is located on the surface of the nanoparticles. Figures 1-6 and 1-7 show that the Fe and N signals, respectively, are located in the core of the iron nitride nanoparticles. Figure 1-8 shows an image of the coated nanoparticles from which the elemental map was obtained.

[0055] Isotropic magnets were made from aluminum oxide-coated iron nitride nanoparticles by uniaxial compression. The nanoparticles were loaded into a 0.25-inch diameter cylindrical die and compressed in a hydraulic press. Figure 1-9 shows the hysteresis loop of a magnet compressed to a pressure of 10 MPa. This magnet has a saturation magnetization of 180 emu / gram and an intrinsic coercivity of 1712 Oe. Figure 1-10 shows the hysteresis loop of a magnet compressed to a pressure of 12 MPa. This magnet has a saturation magnetization of 181 emu / gram and an intrinsic coercivity of 1674 Oe.

[0056] Example 2 The starting material for Example 2 was a total of nine lots of iron nitride nanoparticles, each lot weighing approximately 1.2 grams, prepared in a rotary tube furnace by the same method described in Example 1. The atomic layer deposition parameters in Example 2 were the same as those in Example 1, except for the fluidized bed temperature, which was set at 130°C. The hysteresis loop (Figure 2-1) measured on the coated nanoparticles shows a saturation magnetization of 181 emu / gram and an intrinsic coercivity of 1916 Oe. Again, the X-ray diffraction pattern (Figure 2-2) indicates the presence of α''-Fe 16 N2 phase and ε-Fe 2-3 The major X-ray diffraction peaks of each phase present in Example 2 are identified below in Table 2. Mössbauer spectroscopy reveals a mixture of α-Fe and α-Fe. 16 The mass fraction of the N2 phase is 67%. High-resolution transmission electron microscopy (Figures 2-3 and 2-4) measured the thickness of the aluminum oxide to be 2.0-4.0 nm. Elemental maps (Figures 2-5, 2-6, 2-7, and 2-8) show Al on the surface and Fe and N in the core of the coated iron nitride nanoparticles. Table 2: X-ray diffraction peaks of each phase present in Example 2 [Table 3]

[0057] Figure 2-9 is the hysteresis loop of a 0.25 inch diameter isotropic magnet compressed uniaxially to a pressure of 10 MPa. This magnet has a saturation magnetization of 183 emu / gram and an intrinsic coercivity of 1617 Oe. Figure 2-10 is the hysteresis loop of a magnet compressed to a pressure of 12 MPa. This magnet has a saturation magnetization of 180 emu / gram and an intrinsic coercivity of 1566 Oe.

[0058] Example 3 The starting material for Example 3 was commercially available γ-FeO nanoparticles. Prior to loading into the fluidized-bed reactor, the γ-FeO nanoparticles were sieved to obtain a sieve fraction between 25 μm and 53 μm. After loading into the fluidized-bed reactor, the γ-FeO nanoparticles were transformed into α-Fe nanoparticles by reduction in a 200 sccm flow of H gas at 315°C for 15 hours. The α-Fe nanoparticles were then transformed into iron nitride nanoparticles by flowing 120 sccm of NH gas at 135°C for 40 hours. An aluminum oxide coating was applied to the surface of the iron nitride nanoparticles by atomic layer deposition using nine cycles of alternating trimethylaluminum and water at 100°C.

[0059] The hysteresis loop (Figure 3-1) for the coated nanoparticles shows a saturation magnetization of 158 emu / g and an intrinsic coercivity of 1798 Oe. The X-ray diffraction pattern (Figure 3-2) shows the α''-Fe 16 N2 phase and ε-Fe 2-3 The results show the desired mixture of N and α-Fe phases, with the presence of iron oxide indicating that the γ-FeO nanoparticles did not completely transform to α-Fe prior to nitridation. The major X-ray diffraction peaks for each phase present in Example 3, excluding the iron oxide phase, are identified below in Table 3. The thickness of the aluminum oxide was measured to be 1.9-3.6 nm by high-resolution transmission electron microscopy (Figures 3-3 and 3-4). Elemental maps (Figures 3-5, 3-6, 3-7, and 3-8) show Al on the surface and Fe and N in the core of the coated iron nitride nanoparticles. Table 3: X-ray diffraction peaks of each phase present in Example 3 [Table 4]

[0060] Example 4 The starting material for Example 4 was commercially sourced γ-Fe2O3 nanoparticles. The γ-Fe2O3 nanoparticles were supplied as a dry, agglomerated powder. The γ-Fe2O3 nanoparticle agglomerates had a typical size distribution of 1 to 500 μm. Prior to loading into the fluidized-bed reactor, the γ-Fe2O3 nanoparticles were sieved to obtain a sieve fraction of 25 μm to 53 μm. After loading into the fluidized-bed reactor, the γ-Fe2O3 nanoparticles were transformed into α-Fe nanoparticles by reduction in a 200 sccm H2 gas flow at 315 °C for 24 hours. The α-Fe nanoparticles were then transformed into iron nitride nanoparticles by a 120 sccm NH3 gas flow at 130 °C for 60 hours. An aluminum oxide coating was applied to the surface of the iron nitride nanoparticles by atomic layer deposition using nine cycles of alternating trimethylaluminum and water at 100 °C.

[0061] The hysteresis loop (Figure 4-2) for the coated nanoparticles shows a saturation magnetization of 151 emu / g and an intrinsic coercivity of 1756 Oe. The X-ray diffraction pattern (Figure 4-2) shows the α''-Fe 16 N2 phase and ε-Fe 2-3 The X-ray diffraction pattern shows the desired mixture of N and α-Fe phases. No peaks corresponding to iron oxide were observed in the X-ray diffraction pattern. The major X-ray diffraction peaks for each phase present in Example 4 are identified below in Table 4. The thickness of the aluminum oxide was measured to be 1.6-3.2 nm by high-resolution transmission electron microscopy (Figures 4-3 and 4-4). Elemental maps (Figures 4-5, 4-6, 4-7, and 4-8) show Al on the surface and Fe and N in the core of the coated iron nitride nanoparticles. Table 4: X-ray diffraction peaks of each phase present in Example 4 [Table 5]

[0062] Example 5 The starting material for Example 5 was iron nitride nanoparticles prepared by reduction and nitridation of commercially sourced γ-FeO nanoparticles in a 2-inch diameter rotary tube furnace. A total of 27 lots of nanoparticles, each lot approximately 0.3 grams in mass, were combined together to form the starting material for the aluminum oxide coating operation. First, the γ-FeO nanoparticles were transformed into α-Fe nanoparticles by reduction in flowing H gas at 100 sccm at 330°C for 5 hours. The α-Fe nanoparticles were then transformed into iron nitride nanoparticles by flowing NH gas at a rate of 60°C for 22 hours at 135°C. The iron nitride nanoparticles were removed and stored in a nitrogen-filled glove box. Phase analysis by X-ray diffraction revealed that the iron nitride nanoparticles were α''-Fe. 16 N2 and ε-Fe 2-3 The phase fraction was a mixture of α''-Fe and N. 16 It was N2.

[0063] A fluidized-bed reactor was loaded with 10 grams of iron nitride nanoparticles in a N2-filled glovebox. An aluminum oxide coating was applied to the surface of the iron nitride nanoparticles by atomic layer deposition using nine cycles of alternating trimethylaluminum and water at 200 °C. The coated nanoparticles were removed in air and characterized.

[0064] The hysteresis loop (Figure 5-1) for the coated nanoparticles shows a saturation magnetization of 143 emu / g and an intrinsic coercivity of 787 Oe. The X-ray diffraction pattern (Figure 5-2) shows that the iron nitride nanoparticles are α''-Fe 16 N2 and ε-Fe 2-3The phase fractions were predominantly α-Fe. The major X-ray diffraction peaks for each phase present in Example 5 are identified below in Table 5. Quantitative phase analysis by Mössbauer method revealed that the α-Fe 16 These analyses show that the mass fraction of the N2 phase is 18%. 16 The N2 phase is decomposed to form α-Fe and ε-Fe 2-3 This indicates that the α''-Fe is transformed into N. 16 The decomposition of the N2 phase to α-Fe is believed to cause the decrease in intrinsic coercivity of Example 5 relative to the other examples. Table 5: X-ray diffraction peaks of each phase present in Example 5 [Table 6]

[0065] The thickness of the aluminum oxide coating was measured to be 3.0-5.0 nm by high-resolution transmission electron microscopy (Figures 5-3 and 5-4). Elemental maps (Figure 5-5) show that Al is located on the surface of the nanoparticles. Furthermore, elemental maps (Figures 5-6 and 5-7) show that Fe and N are sequestered in the core of the nanoparticles, and more specifically, α''-Fe. 16 Figures 5-8 are images of the coated nanoparticles from which elemental maps were obtained, showing evidence of decomposition of the N2 phase.

[0066] Figure 5-9 is the hysteresis loop of a 0.25 inch diameter isotropic magnet compressed uniaxially to a pressure of 10 MPa. This magnet has a saturation magnetization of 150 emu / gram and an intrinsic coercivity of 681 Oe. Figure 5-10 is the hysteresis loop of a magnet compressed to a pressure of 12 MPa. This magnet has a saturation magnetization of 148 emu / gram and an intrinsic coercivity of 677 Oe. The coercivity of the magnet is due to the α''-Fe 16 The coercive force of the magnets produced in Examples 1 and 2 is smaller due to the decomposition of the N2 phase.

[0067] Considering these results, nanoparticles suitably provided herein are characterized as having X-ray diffraction patterns substantially similar to those shown in Figures 1-2, 2-2, or 3-2. In addition, the intensity ratio of X-ray diffraction peaks can be used as a "fingerprint" of acceptable phase fractions of nanoparticles provided herein. For example, the ratio of the α-Fe(211) peak (at about 82.2° 2θ) / α''-Fe(211) peak (at about 42.7° 2θ) 16 The ratio of the integrated intensities of the N2(202) peaks is desirably 0.20 or less, more preferably 0.15 or less. The X-ray diffraction patterns and diffraction angles provided herein were measured using an X-ray source having a Cu target configured to emit single-wavelength Cu-Kα X-rays.

[0068] α''-Fe for each nanoparticle 16 The grain size (core thickness) of the N2 phase is calculated by the α''-Fe 16 The core thicknesses were calculated using the Scherrer equation applied to the X-ray diffraction peak widths of N2. Peak parameters were determined using the Rietveld refinement method, calibrated for the instrumental broadening of the diffractometer. The core thicknesses are listed in Table 6.

[0069] A summary of the properties and characteristics of the aluminum oxide coated iron nitride nanoparticles made according to the examples is provided below in Table 6. Table 6: Properties of aluminum oxide coated iron nitride nanoparticles [Table 7]

[0070] A summary of the magnetic properties of isotropic magnets produced by uniaxial compression of aluminum oxide coated iron nitride nanoparticles is provided below in Table 7. Table 7: Magnetic properties of isotropic magnets [Table 8]

[0071] A summary of the quantitative analysis of the mass fractions of the layers present in Examples 1, 2 and 5, as determined by the Mössbauer method, is provided below in Table 8. Table 8: Mass fractions of phases determined by Mössbauer method [Table 9]

[0072] When ranges are used herein for physical properties, such as molecular weight, or chemical properties, such as chemical formula, all combinations and subcombinations of ranges for specific embodiments therein are intended to be included.

[0073] The disclosures of each patent, patent application and publication cited or described in this application are hereby incorporated by reference in their entirety.

[0074] Those skilled in the art will recognize that many changes and modifications can be made to the preferred embodiments of the present invention and that such changes and modifications can be made without departing from the spirit of the present invention. It is, therefore, intended by the appended claims to cover all such equivalent variations that fall within the true spirit and scope of the present invention.

[0075] Item 1. 50% or more by weight of α''-Fe 16 1. A plurality of iron nitride nanoparticles comprising: an iron-based core including an N2 phase; and a thin material disposed adjacent to and on the iron-based core, the thin material configured to enable nitridation of the iron-based core.

[0076] Item 2. A plurality of iron nitride nanoparticles according to item 1, wherein the thin material comprises a non-ferromagnetic material.

[0077] Item 3. The plurality of iron nitride nanoparticles of item 2, wherein the non-ferromagnetic material comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof.

[0078] Item 4. A plurality of iron nitride nanoparticles according to item 1, wherein the thin material is configured as a discontinuous shell.

[0079] Item 5. A plurality of iron nitride nanoparticles according to item 1, wherein the thin material is configured as a substantially continuous shell through which nitrogen can diffuse.

[0080] Item 6. A plurality of iron nitride nanoparticles according to item 1, characterized in that they have an Msat greater than about 145 emu / g and a coercivity greater than about 1000 Oe.

[0081] Item 7. The α''-Fe nanoparticles 16 2. The plurality of iron nitride nanoparticles of claim 1, wherein the mass fraction of N2 phase is characterized by the Mössbauer method as being greater than 60%.

[0082] Item 8. A plurality of nanoparticles comprising an iron nitride core and a thin non-ferromagnetic material disposed adjacent to the core, the nanoparticles comprising at least 50% α''-Fe by weight. 16 A plurality of nanoparticles comprising an N2 phase.

[0083] Item 9. The plurality of nanoparticles of item 8, wherein the average thickness of the non-ferromagnetic thin material is less than 5 nm.

[0084] Item 10. A plurality of nanoparticles according to item 8, characterized in that they have an Msat greater than about 145 emu / g and a coercivity greater than about 1000 Oe.

[0085] Item 11. When characterized by X-ray diffraction using an X-ray source having a Cu target configured to emit single-wavelength Cu-Kα X-rays, the X-ray diffraction shows an α-Fe(211) peak (at 2θ of 82.2°) / α''-Fe(211) peak (at 2θ of 42.7°). 16 A plurality of nanoparticles giving rise to diffraction peaks in which the ratio of the integrated intensity of the N2(202) peak to that of the N2(202) peak is 0.2 or less.

[0086] Item 12. α''-Fe: 50% or more by weight of magnetic material 16 A magnetic material comprising: a plurality of iron nitride nanophases, each of the nanophases comprising an N2 phase; and a non-ferromagnetic material disposed between the nanophases.

[0087] Item 13. The magnetic material of item 12, wherein the non-ferromagnetic material disposed between the nanophases comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof.

[0088] Item 14. The magnetic material of item 12, characterized in that it has an Msat of greater than about 160 emu / g and a coercivity of greater than about 700 Oe.

[0089] Item 15. The magnetic material of item 12, wherein the thickness of the non-ferromagnetic material disposed between the nanophases has an average thickness of less than 5 nm.

[0090] Item 16. 50% or more by weight of α''-Fe 16 1. A plurality of iron nitride nanoparticles comprising: an iron-based core comprising an N2 phase; and a shell on the iron-based core comprising a material characterized as being a ductile metal, a non-ferromagnetic material, or both.

[0091] Item 17. The iron nitride nanoparticles of item 16, wherein the core has a major dimension of 20 nm to 200 nm.

[0092] Item 18. The iron nitride nanoparticles of item 16, wherein the core has a major dimension of 40 nm to 80 nm.

[0093] Item 19. Iron nitride nanoparticles according to item 16, wherein the shell has a thickness of 0.1 nm to 5 nm.

[0094] Item 20. Iron nitride nanoparticles according to item 16, wherein the shell has a thickness of 0.1 nm to 4 nm.

[0095] Item 21. Iron nitride nanoparticles according to item 16, wherein the shell has a thickness of 0.1 nm to 3 nm.

[0096] Item 22. The iron nitride nanoparticles of item 16, wherein the nanoparticles are spherical.

[0097] Item 23. The iron nitride nanoparticles of item 16, wherein the nanoparticles are non-spherical.

[0098] Item 24. Iron nitride nanoparticles according to item 16, wherein the nanoparticles have a porous structure.

[0099] Item 25. The iron nitride nanoparticles of item 16, wherein the nanoparticles are in the form of rods, needles, prolate spheroids, oblate spheroids, discs and plates, foams, gels, or sponges.

[0100] Item 26. The core is made of α-Fe, α'-FeN, γ'-FeN and ε-Fe 2-3 17. The iron nitride nanoparticles of paragraph 16, further comprising one or more phases of N.

[0101] Item 27. The iron nitride nanoparticles of item 16, wherein the shell further comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, or any combination thereof.

[0102] Item 28. The iron nitride nanoparticles of item 16, wherein the ductile metal comprises Cu, Al, Sn, Zn, or any combination thereof.

[0103] Item 29. A bulk magnet comprising a plurality of iron nitride nanoparticles according to any one of the preceding items.

[0104] Item 30. A bulk magnet according to item 29, wherein the saturation magnetization value (Msat) of the bulk magnet is 180 emu / g or more.

[0105] Item 31. A bulk magnet according to item 29, wherein the saturation magnetization value (Msat) of the bulk magnet is 195 emu / g or more.

[0106] Item 32. A bulk magnet according to item 29, wherein the bulk has a coercive force value (Hci) of 1500 Oe or more.

[0107] reference Ahvenniemi, E. et al., “Review Article: Recommended reading list of early publications on atomic layer deposition-Outcome of the “Virtual Project on the History of ALD”,” Journal of Vacuum Science & Technology A: Vacuum,Surfaces,and Films, Vol.35, 2016, 010801 King, DM et al., "Atomic layer deposition on particles using a fluidized bed reactor with in situ mass spectrometry", Surface & Coatings Technology, Vol. 201, 2007, pp 9163-71 LFHakim, LF et al., “Synthesis of oxidation-resident metal nanoparticles via atomic layer deposition”, Nanotechnology, Vol. 18, (2007), 354093 Sankar et al., "Iron nitride powders for use in magnetic, electromagnetic, and microelectronic devices," U.S. Patent No. 8,535,634, September 17, 2013 The following embodiments can be given as examples of the present invention. (Appendix 1) a. 50% or more by weight of α''-Fe 16 with an iron-based core containing N2 phase; b. a thin material disposed on and adjacent to the iron-based core, the thin material configured to enable nitriding of the iron-based core; A plurality of iron nitride nanoparticles comprising: (Appendix 2) 10. The plurality of iron nitride nanoparticles of claim 1, wherein the thin material comprises a non-ferromagnetic material. (Appendix 3) 3. The plurality of iron nitride nanoparticles of claim 2, wherein the non-ferromagnetic material comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof. (Appendix 4) 2. The plurality of iron nitride nanoparticles of claim 1, having an Msat greater than about 145 emu / g and a coercivity greater than about 1000 Oe. (Appendix 5) A plurality of nanoparticles comprising an iron nitride core and a thin non-ferromagnetic material disposed adjacent to the core, the nanoparticles comprising at least 50% α″-Fe by weight. 16 A plurality of nanoparticles comprising an N2 phase. (Appendix 6) 6. The plurality of nanoparticles of claim 5, wherein the non-ferromagnetic thin material has an average thickness of less than 5 nm. (Appendix 7) 6. The nanoparticles of claim 5, having an Msat greater than about 145 emu / g and a coercivity greater than about 1000 Oe. (Appendix 8) When characterized by X-ray diffraction using an X-ray source having a Cu target configured to emit single wavelength Cu-Kα X-rays, the X-ray diffraction shows an α-Fe(211) peak (at about 82.2° 2θ) / α''-Fe(211) peak (at about 42.7° 2θ). 16 A plurality of nanoparticles giving rise to diffraction peaks in which the ratio of the integrated intensity of the N2(202) peak to that of the N2(202) peak is 0.2 or less. (Appendix 9) a. α''-Fe: 50% or more by weight of magnetic material 16 a plurality of iron nitride nanophases, characterized by including an N2 phase; b. a non-ferromagnetic material disposed between the nanophases; a magnetic material comprising: (Appendix 10) 10. The magnetic material of claim 9, wherein the non-ferromagnetic material disposed between the nanophases comprises FeO, α-Fe2O3, ZnO, Al2O3, SiO2, TiO2, ZrO2, CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof. (Appendix 11) 50% or more by weight of α''-Fe 16 with an iron-based core containing N2 phase; a shell on said iron-based core comprising a material characterized as a ductile metal, a non-ferromagnetic material, or both; A plurality of iron nitride nanoparticles comprising: (Appendix 12) 12. The iron nitride nanoparticles of claim 11, wherein the core has a major dimension of 20 nm to 200 nm. (Appendix 13) 12. The iron nitride nanoparticles according to claim 11, wherein the shell has a thickness of 0.1 nm to 5 nm. (Appendix 14) 12. The iron nitride nanoparticles of claim 11, wherein the ductile metal comprises Cu, Al, Sn, Zn, or any combination thereof. (Appendix 15) A bulk magnet comprising a plurality of iron nitride nanoparticles according to any one of claims 1 to 14. (Appendix 16) The saturation magnetization value (Msat 16. The bulk magnet according to claim 15, wherein the densitometric coefficient of porosity is 180 emu / g or more. (Appendix 17) The coercive force value of the bulk magnet (H ci 16. The bulk magnet according to claim 15, wherein the magnet strength is 1500 Oe or more.

Claims

1. a. 50% or more by weight of α''-Fe １６ N ２ an iron-based core containing a phase; b. a thin material disposed on and adjacent to the iron-based core, the thin material configured to enable nitriding of the iron-based core; and the thin material is Al ２ O ３ wherein the thin material has an average thickness of 0.1 to 3.0 nm and has an Msat greater than 180 emu / g and a coercivity greater than 2000 Oe.

2. The plurality of iron nitride nanoparticles of claim 1 , wherein the thin material further comprises a non-ferromagnetic material.

3. The non-ferromagnetic material is FeO, α-Fe ２ O ３ , ZnO, SiO ２ , TiO ２ , ZrO ２ 3. The plurality of iron nitride nanoparticles of claim 2, comprising CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn, or any combination thereof.

4. A plurality of nanoparticles comprising an iron nitride core and a thin non-ferromagnetic material disposed adjacent to the core, the nanoparticles having at least 50% α″-Fe by weight. １６ N ２ phase, and the non-ferromagnetic thin material is ２ O ３ wherein the non-ferromagnetic thin material has an average thickness of 0.1 to 3.0 nm and has an Msat greater than 180 emu / g and a coercivity greater than 2000 Oe.

5. When characterized by X-ray diffraction using an X-ray source having a Cu target configured to emit single wavelength Cu-Kα X-rays, the X-ray diffraction shows an α-Fe(211) peak (at 2θ of 82.2°) / α″-Fe(211) peak (at 2θ of 42.7°). １６ N ２ 10. The plurality of nanoparticles of claim 1, which produce a diffraction peak having a ratio of integrated intensities of the (202) peaks of 0.2 or less.

6. a. α''-Fe: 50% or more by weight of the magnetic material １６ N ２ a plurality of iron nitride nanophases, characterized in that they comprise a phase; b. a non-ferromagnetic material disposed between the nanophases; and the non-ferromagnetic material is Al ２ O ３ wherein the non-ferromagnetic material has an average thickness of 0.1 to 3.0 nm, and has an Msat greater than 180 emu / g and a coercivity greater than 2000 Oe.

7. The non-ferromagnetic material disposed between the nanophases is FeO, α-Fe ２ O ３ , ZnO, SiO ２ , TiO ２ , ZrO ２ 7. The magnetic material of claim 6, further comprising CoO, NiO, Mn, Cr, CrN, MnN, Cu, Al, Sn, Zn or any combination thereof.

8. 50% or more by weight of α''-Fe １６ N ２ an iron-based core containing a phase; a shell on said iron-based core comprising a material characterized as a ductile metal, a non-ferromagnetic material, or both; wherein the shell is Al ２ O ３ wherein the shell has an average thickness of 0.1 to 3.0 nm, and has an Msat greater than 180 emu / g and a coercivity greater than 2000 Oe.

9. 9. The iron nitride nanoparticles of claim 8, wherein the longest dimension of the core is between 20 nm and 200 nm.

10. 9. The iron nitride nanoparticles of claim 8, wherein the ductile metal comprises Cu, Al, Sn, Zn, or any combination thereof.

11. A bulk magnet comprising a plurality of nanoparticles according to any one of claims 1 to 5 and 8 to 10.

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