Method for the production of hard / soft magnetic FeCo / SiO2 / MnBi nanoparticles with magnetically induced morphology and hard / soft magnetic FeCo / SiO2 / MnBi nanoparticles with magnetically induced morphology

Abstract

Method for the production of a core-shell-shell FeCo / SiO2 / MnBi nanoparticle comprising: a) Joint reduction of an iron ion and a cobalt ion from a common solution; and joint precipitation of an FeCo alloy nanoparticle; Isolating the FeCo nanoparticle from the reduction mixture; b) Forming a silicon dioxide coating on the FeCo nanoparticle to obtain a core-shell nanoparticle, wherein the nanoparticles are treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a basic catalyst; and c) Forming a MnBi alloy nanocoating on the core-jacket nanoparticle by reducing Bi ions through a Mn-lithium borohydride complex by precipitation from a solution as a MnBi alloy onto the silicon dioxide jacket; the formation of the MnBi alloy nanocoating c) is carried out in a magnetic field of 0.005 T (50 Gauss) to 0.08 T (800 Gauss).

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The present invention relates to a method for producing magnetic core-cladding-cladding nanoparticles comprising a core of an iron-cobalt alloy, a silicon dioxide intermediate cladding, and a surface layer of a manganese-bismuth alloy on the silicon dioxide cladding, which exhibits a modified morphology obtained by nanoparticle synthesis in an applied magnetic field. This method further offers the possibility of producing magnetic nanoparticles that can be designed with specific properties, thus providing a nanoparticle material suitable for producing a permanent magnet that represents a rare-earth-free alternative to the standard neodymium-iron borate permanent magnet material. Discussion of the background

[0002] The inventors are currently conducting a research program investigating both soft and hard magnetic materials derived from nanoparticulate materials obtained through wet-chemical synthesis processes. For example, patent application US 2015 / 0 068 646 A1, filed on September 12, 2013, discloses MnBi nanoparticles with a particle size of 5 to 200 nm as a source of hard magnetic materials. Furthermore, patent application US 2015 / 0 294 775 A1, filed on April 14, 2014, discloses core-jacket nanoparticles comprising an iron-cobalt nanoparticle core of less than 200 nm with a silicon dioxide jacket and a metal-silicate interface as a source of soft magnetic materials. In addition, the inventors have disclosed in patent application 6, 2015, that the following materials are also available:Disclosure specification US 2015 / 0325346A1, filed in May 2014, discloses core-cladding-cladding nanoparticles comprising a soft magnetic nanoparticle core made of an iron-cobalt alloy, a first silicon dioxide cladding on the core, and a further hard magnetic nanocoating made of a manganese-bismuth alloy.

[0003] The published patent applications US 2012 / 0208026A1, US 2004 / 0134565A1 and US 2014 / 0027667A1 disclose magnetic nanoparticles and their manufacturing processes according to the prior art.

[0004] Magnetic materials can generally be divided into two classes: permanently magnetizable hard magnetic materials and soft magnetic materials, whose magnetism is reversible under weak applied fields. For soft magnetic materials, it is important to minimize energy loss, usually referred to as "core loss," whereas in hard magnetic materials, changes in magnetization are preferably avoided. High core losses are therefore characteristic of permanent magnetic materials but undesirable in soft magnetic materials.

[0005] Many of today's advancing technologies require an efficient and powerful hard magnet as a fundamental component in the construction of devices. Such devices range from mobile phones to high-performance electric motors, and considerable efforts are continuously being made across the board to find materials that not only meet current requirements but also satisfy the ever-increasing demand for efficient, more cost-effective, and easier-to-manufacture hard magnetic materials.

[0006] Traditionally, neodymium-iron borate is generally considered one of the strongest and most efficient hard magnetic materials available. However, since this material is based on the rare-earth element neodymium, it is expensive and the available supply is often unstable. Consequently, there is a need for a material that performs as well as or better as neodymium-iron borate as a hard magnet, yet is based on materials with readily available and more cost-effective components.

[0007] Magnetic device components are manufactured from powders by compacting the powders into a defined shape and then sintering the resulting compact at temperatures of 200°C or higher. Sintering the component after compaction is necessary to achieve satisfactory mechanical properties by creating bonds between the particles and consequently ensuring strength.

[0008] Technological advances in all areas of the communications and energy generation sectors require increasingly powerful magnetic powders with controllable or adjustable magnetic properties, enabling the production of customized, cost-effective and readily available magnetic components.

[0009] One object of the present invention is therefore to provide a method for producing nanoparticulate powders with magnetic properties that are adjustable according to the controllable variables of the method. Summary of the invention

[0010] These and other problems were solved according to the present invention, the first embodiment of which includes a method for producing a core-cladding-cladding FeCo / SiO2 / MnBi nanoparticle comprising: a) joint or simultaneous reduction of an iron ion and a cobalt ion from a common solution; and joint or simultaneous precipitation of an FeCo alloy nanoparticle; isolation of the FeCo nanoparticle from the reduction mixture; b) Forming a silicon dioxide coating on the FeCo nanoparticle to obtain a core-shell nanoparticle, wherein the nanoparticles are treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a basic catalyst; and c) Forming a MnBi alloy nanocoating on the core-jacket nanoparticle by reducing Bi with a Mn reagent to precipitate from a solution as a MnBi alloy onto the silicon dioxide jacket, wherein the formation of the MnBi alloy nanocoating c) is carried out in a magnetic field of 0.005 to 0.08 T (50 to 800 Gauss).

[0011] In another embodiment, the present invention includes the core-jacket-jacket nanoparticle obtained according to the method of the first embodiment.

[0012] In one aspect of this embodiment, the width of the MnBi coating can be between 0.5 and 200 nm.

[0013] The preceding paragraphs are intended as a general introduction and are not meant to limit the scope of the subsequent claims. The preferred embodiments presented here, along with further advantages, are best understood by referring to the following detailed description in conjunction with the accompanying drawings. Brief description of the drawings Fig. Figure 1 shows a TEM image of the core-mantle-mantle nanoparticles produced in the example. Fig. Figure 2 shows a sampling of DDK and M(T) data for the core-jacket-jacket nanoparticles produced in the example. Here, 1 emu corresponds to 10^-3 J / T. Fig. Figure 3 shows a Z-contrast TEM image of the core-mantle-mantle nanoparticles produced in the example. Fig. Figure 4 shows a comparison between the Z-contrast TEM image of the core-mantle-mantle nanoparticles obtained in Example II and the core-mantle-mantle nanoparticles obtained in Example I. Detailed description of the invention

[0014] Unless otherwise stated, all areas described in this description include all values ​​and sub-areas therein.

[0015] Additionally, the indefinite article “ein” or “eine” throughout the description has the meaning “one or more”, unless otherwise stated.

[0016] In an ongoing investigation of magnetic materials, and in particular nanoparticulate magnetic materials, the present inventor has identified a manganese-bismuth alloy in nanoparticulate form as a material of potential use as a replacement for neodymium-iron borate in the production of permanent magnets. The formation of high coercive field strengths of up to 4 T has been predicted for MnBi nanoparticles. The invention disclosed in patent application US 2015 / 0068646A1, filed on September 12, 2013, discloses some results of that work.

[0017] The inventors are also currently conducting ongoing investigations with soft magnetic nanoparticulate materials, such as those in patent application US 2015 / 0294775A1, filed on April 14, 2014, which discloses core-jacket nanoparticles having an iron-cobalt nanoparticle core of less than 200 nm with a silicon dioxide jacket and a metal-silicate interface.

[0018] In ongoing research with these and other systems, the inventors have surprisingly discovered that core-cladding nanoparticles, obtained by applying a manganese-bismuth nanocoating to a core-cladding nanoparticle with an FeCo alloy core and a silicon dioxide coating, provide a material with magnetic properties that are highly tunable according to the relative size and nature of each of the core-cladding components. Such a complex combination of soft and hard magnetic components within a single nanoparticle is novel and offers numerous opportunities for the discovery and development of new magnetic materials and devices.

[0019] In a first embodiment, the present invention includes a method for producing a core-cladding-cladding FeCo / SiO2 / MnBi nanoparticle, comprising: a) joint or simultaneous reduction of an iron ion and a cobalt ion from a common solution; and joint or simultaneous precipitation of an FeCo alloy nanoparticle; isolation of the FeCo nanoparticle from the reduction mixture; b) Forming a silicon dioxide coating on the FeCo nanoparticle to obtain a core-shell nanoparticle, wherein the nanoparticles are treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a basic catalyst; and c) Forming a MnBi alloy nanocoating on the core-jacket nanoparticle by reducing Bi with a Mn reagent to precipitate from a solution as a MnBi alloy onto the silicon dioxide jacket; wherein the formation of the MnBi alloy nanocoating c) is carried out in a magnetic field of 0.005 to 0.08 T (50 to 800 Gauss).

[0020] The inventors have discovered that the formation of individual FeCo alloy nanoparticles coated with silicon dioxide shells of varying thicknesses can be achieved via a scalable wet-chemical process. Surprisingly, they found that the formation of interfacial metal silicates can significantly alter the nanomagnetism in these ultra-high surface area FeCo alloy nanoparticle systems. Evidence of the formation of an interfacial metal silicate layer was obtained from X-ray photoelectron spectra obtained via the Fe and Co-2p transitions. Furthermore, increasing the thickness of the silicon dioxide shell (by changing the duration of the silicon dioxide reaction) resulted in the formation of a thicker interfacial metal silicate layer, thereby increasing the overall magnetic anisotropy of the nanoparticles, as evidenced by elevated blocking temperatures and altered coercive field strengths.The inventors have thus surprisingly discovered that by producing superparamagnetic iron-cobalt alloy nanoparticles encapsulated in silicon dioxide shells with varying wet synthesis treatment durations, core-shell FeCo nanoparticles with different nanomagnetic properties can be obtained. In certain embodiments, the diameter of the iron-cobalt alloy nanoparticle core is 100 nm or less, and in other embodiments, the diameter of the iron-cobalt alloy nanoparticle core is between 2 nm and 50 nm.

[0021] According to the invention, the iron-cobalt alloy nanoparticle grains have the same or approximately the same size as the magnetic domain of a single iron-cobalt alloy particle and are therefore superparamagnetic. Although the inventors do not limit themselves to theory, they assume that controlling the grain size so that it approximately corresponds to that of the particles' magnetic domain is a factor that contributes to the reduced hysteresis of a magnetic core according to the present invention. Furthermore, the presence of insulating silicon dioxide sheaths around the core grains is a factor that contributes to the low formation of eddy currents in a magnetic core according to the present invention.

[0022] It is conventionally known that the particle size range in which single-domain particles exhibit superparamagnetism has an upper limit that is characteristic of the chemical composition of the particles.

[0023] The inventors discovered that a thin metal silicate interface is formed simultaneously during the synthesis of the silicon dioxide shell. Evidence of the metal silicate interface layer formation was obtained from X-ray photoelectron spectra obtained via the Fe and Co-2p transitions. When the thickness of the silicon dioxide shell was increased (by varying the duration of the silicon dioxide reaction), a thicker metal silicate interface layer formed, thereby increasing the overall magnetic anisotropy of the nanoparticles, as evidenced by elevated blocking temperatures and altered coercive field strengths. The inventors recognized that understanding the effect of this metal silicate interface layer on controlling magnetic properties is crucial for effectively utilizing these materials in applications such as low-loss transformer cores.

[0024] In an investigation of core-cladding nanoparticles made from an FeCo alloy, the inventors discovered that interfacial metal silicates formed during the synthesis of the silicon dioxide cladding coating alter the overall magnetic anisotropy of the nanoparticles as a phase of higher anisotropy, which is a combination of Fe- and Co-based silicates with an effect of increasing the “magnetically active volume” of the nanoparticles compared to a mere FeCo nanoparticle.

[0025] Samples of nanoparticles made from a binary FeCo alloy with a single magnetic domain were synthesized (see example), except that the duration of the SiO2 reaction times was varied, resulting in SiO2 sheaths of different thicknesses: a reaction time of 1 minute yielded a 3 nm thick sheath, 10 minutes a 4 nm thick sheath, and 20 minutes a 6 nm thick sheath. The average diameter of the FeCo nanoparticles and the thickness of the SiO2 sheath were determined, and for all three samples of core-sheath nanoparticles (FeCo / SiO2 (3 nm), FeCo / SiO2 (4 nm), and FeCo / SiO2 (6 nm)), an average FeCo core diameter of 4 ± 1 nm was found, indicating a high degree of reproducibility in the synthesis of the nanoparticle core. The thickness of the silicon dioxide coatings was determined in a similar manner and yielded 3 ± 1 nm, 4 ± 1 nm and 6 ± 1 nm for the FeC samples. O / SiO2(3 nm), FeCo / SiO2(4 nm), and FeCo / SiO2(6 nm). TEM images revealed that the FeCo cores were completely covered by the silicon dioxide shells. X-ray diffraction pattern analysis indicated the presence of both Fe and Co silicates. However, the relative proportions appear to fluctuate, and the inventors hypothesize, without limiting themselves to theory, that the metal silicate content may be related to the thermodynamic energy of the metal silicate formation. Investigations showed that Fe and Co silicates formed at the interface between the FeCo nanoparticle core and the SiO2 shell during the synthesis process. The relative integrated areas of the Fe 0 - and Co 0 However, metal peaks of the different core-jacket nanoparticle systems indicated that Fe silicates may be preferentially formed over Co-silicates.

[0026] FeCo / SiO2 nanoparticles can be synthesized by the ethanolic reaction of sodium borohydride with iron dichloride and cobalt dichloride in a solution of sodium hydroxide and tetraoctylammonium bromide. The resulting nanoparticles can be treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a basic catalyst to form silicon dioxide shells. These particles can then be purified using an aqueous ethanol rinse.

[0027] As indicated, the length of the FeCo nanoparticle treatment determines the width of the silicon dioxide coating and, consequently, the width of the metal silicate layer. The longer the treatment time, the greater the coating quantity and the wider the metal silicate layer.

[0028] The synthesis can be carried out for a time necessary to produce a metal silicate layer of 0.5 to 20 nm, preferably 0.8 to 10 nm and most preferably 1.0 to 8 nm.

[0029] According to the invention, the manganese-bismuth coating on the FeCo-silicon dioxide core-cladding nanoparticle is formed under the influence of an applied magnetic field. The strength of the magnetic field can be 0.001 to 0.1 T (10 to 1000 Gauss), preferably 0.025 to 0.09 T (25 to 900 Gauss), and most preferably 0.005 to 0.08 T (50 to 800 Gauss). The source of the magnetic field is not restricted and can be formed, for example, by placing the reaction mixture near a permanent magnet, next to an electromagnet, between two permanent magnets, or by placing the reaction mixture in a solenoid.

[0030] The manganese-bismuth alloy coating can be formed by a process that, in the presence of the FeCo-silicon dioxide core-shell nanoparticles, comprises: milling a Mn powder with a hydride reducing agent in a ball mill; adding, while stirring, a solution of a bismuth salt, a long-chain carboxylate, and an alkylamine to the Mn hydride reducing agent in an ether solvent; after completion of the addition of the bismuth salt solution; and continuing to stir to form the core-shell-shell FeCo / SiO2 / MnBi nanoparticles.

[0031] The ether solvent for the hydride treatment can be any ether compatible with hydride reaction conditions. Suitable ether solvents include, but are not limited to, tetrahydrofuran (THF), 2-methyltetrahydrofuran, diethyl ether, diisopropyl ether, 1,4-dioxane, dimethoxyethane, diethylene glycol diethyl ether, 2-(2-methoxyethoxy)ethanol, and methyl tert-butyl ether. THF may be a preferred solvent.

[0032] The hydride reducing agent can be any material capable of reacting with the manganese, including NaH, LiH, CaH2, LiAlH4, and LiBH4. LiBH4 can be a preferred hydride treatment agent. According to the invention, a preferred hydride can be one that forms a reagent complex between manganese and LiBH4.

[0033] The hydride treatment can involve milling manganese powder with lithium borohydride powder for 4 hours in a planetary ball mill at 150 to 400 rpm. Variations of these conditions can be optimized to appropriately yield an ideal manganese and lithium borohydride complex.

[0034] Furthermore, the stoichiometric ratio of hydride to Mn can vary from 1 / 1 to 100 / 1.

[0035] The bismuth can be added in any ether-soluble salt form and is preferably added as a salt of a long-chain carboxylic acid. In a preferred embodiment, the bismuth is added as bismuth neodecanoate. The molar ratio of bismuth to manganese can vary from 0.8 / 1 to 1.2 / 1. Preferably, the ratio of bismuth to manganese is from 0.9 / 1 to 1.1 / 1, and most preferably, the ratio of bismuth to manganese is 1 / 1. The addition time of the bismuth compound can be varied to optimize and modify the size and properties of the MnBi coating. The coating width can be from 0.5 to 200 nm, preferably from 1.0 to 100 nm, and most preferably from 2 to 20 nm. Preferably, the addition time is less than one hour, and in a preferred embodiment, the addition time is about 20 minutes.

[0036] The addition of the bismuth compound may optionally introduce an organic amine, preferably a primary amine with a carbon chain of 6 to 12 carbons, into the reaction mixture to achieve a smaller size of the coated core-shell-shell nanoparticles. The resulting solids can be removed from the mother liquor and washed with water to remove soluble impurities.

[0037] As in Fig. As stated in Figure 2, both the soft-phase FeCo and the hard-phase MnBi glow at temperatures characteristic of FeCo and MnBi, respectively, when the core-cladding-cladding nanoparticles of the invention are thermally treated in an annealing process.

[0038] Following the general description of this invention, a more detailed understanding can be obtained by referring to certain specific examples, which are provided here only for illustrative purposes and are not intended to represent a limitation unless otherwise stated. Those skilled in the art will recognize the usefulness of the devices of the present invention as a battery, as well as the general benefit of the electrolyte system described herein. EXAMPLES. Production of core-jacket-jacket iron-cobalt / silicon dioxide / manganese-bismuth nanoparticles without applying a magnetic field. a) 0.489 g sodium hydroxide, 12.892 g tetraoctylammonium bromide, 10.922 g iron dichloride tetrahydrate, and 12.042 g cobalt chloride hexahydrate were dissolved in 250 ml ethanol and placed under argon. A solution of 12.258 g sodium borohydride dissolved in 450 ml ethanol was then added to the iron-cobalt mixture. After the addition of the borohydride was complete, the reaction mixture was diluted with 100 ml water. The product, the FeCo nanoparticles, was then washed with a 70% water / 30% ethanol solution. b) The FeCo nanoparticles were then suspended in a mixture of 625 ml water and 2 ml triethylamine. A solution of 0.5 ml tetraethyl orthosilicate in 390 ml ethanol was then added to the FeCo suspension, and the resulting mixture was allowed to react for 15 minutes to obtain silicon dioxide-coated nanoparticles. The coated nanoparticles were then washed with ethanol. c) The silicon dioxide-coated FeCo nanoparticles (0.27 g) were suspended in 200 ml of THF. 0.152 g of heptyl cyanide, 0.008 g of lithium borohydride, and 0.012 g of Mn(LiBH4)2 were added to the FeCo nanoparticle suspension. A solution of 0.082 g of bismuth neodecanoate in 15 ml of THF was then added dropwise to the stirred suspension. The product was finally washed with THF.

[0039] A TEM image of the fabricated core-mantle-mantle nanoparticles is in Fig. 1 shown.

[0040] The Z-contrast TEM image of the Fig. Figure 3 shows how the MnBi phase exhibits an island distribution throughout the entire FeCo / SiO2.

[0041] Fig. Figure 2 shows that DDK and M(T) data reveal temperature-associated annealing characteristics for the FeCo and MnBi nanophases. II. Production of core-shell-shell iron-cobalt / silicon dioxide / manganese-bismuth nanoparticles under the application of a magnetic field

[0042] FeCo nanoparticles coated with silicon dioxide were produced exactly according to a) and b) of Example I.

[0043] Then, in step c), the silicon dioxide-coated FeCo nanoparticles (0.27 g) were suspended in 200 ml of THF. 0.152 g of heptyl cyanide, 0.008 g of lithium borohydride, and 0.012 g of Mn(LiBH4)2 were added to the FeCo nanoparticle suspension. The reaction flask containing the suspension was placed within several permanent magnets to apply a magnetic field of 0.055 T (550 gauss) to the mixture. A solution of 0.082 g of bismuth neodecanoate in 15 ml of THF was then added dropwise to the stirred suspension within the magnetic field. The resulting product was washed with THF as in Example I.

[0044] A in Fig.Figure 4 shows a comparison between the Z-contrast TEM image of the nanoparticles obtained in Example II and the Z-contrast TEM image of the nanoparticles obtained in Example I, revealing a dramatic difference in the distribution of the MnBi alloy, resulting from the MnBi coating process in a magnetic field.

Claims

[1] Method for producing a core-shell-shell FeCo / SiO2 / MnBi nanoparticle comprising: a) Joint reduction of an iron ion and a cobalt ion from a common solution; and joint precipitation of an FeCo alloy nanoparticle; Isolating the FeCo nanoparticle from the reduction mixture; b) Forming a silicon dioxide coating on the FeCo nanoparticle to obtain a core-shell nanoparticle, wherein the nanoparticles are treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a basic catalyst; and c) Forming a MnBi alloy nanocoating on the core-jacket nanoparticle by reducing Bi ions through a Mn-lithium borohydride complex by precipitation from a solution as a MnBi alloy onto the silicon dioxide jacket; the formation of the MnBi alloy nanocoating c) is carried out in a magnetic field of 0.005 T (50 Gauss) to 0.08 T (800 Gauss). [2] Method according to claim 1, wherein the magnetic field is obtained by placing the reaction mixture in the immediate vicinity of a permanent magnet. [3] Method according to claim 1, wherein the magnetic field is obtained by placing the reaction mixture in a solenoid. [4] Core-cladding-cladding FeCo / SiO2 / MnBi nanoparticles obtained by the method of claim 1. [5] Core-shell-shell FeCo / SiO2 / MnBi nanoparticles obtained by the method of claim 1, wherein the relationship between the shell morphology and nanostructure of MnBi on the FeCo / SiO2 core-shell nanoparticle is determined by the applied magnetic field.

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