Hard / soft magnetic MnBi / SiO2 / FeCo nanoparticles

Abstract

Core-mantle-core nanoparticle system comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate shell made of silicon dioxide coating the superparamagnetic core; an outer core made of a manganese-bismuth alloy on the silicon dioxide intermediate shell; and a metal silicate interface layer between the superparamagnetic core and the silicon dioxide mantle; wherein a diameter of the iron-cobalt alloy core is 200 nm or less.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a magnetic nanoaggregation of nanoparticles comprising an iron-cobalt core with a silicon dioxide interlayer and manganese-bismuth alloy nanoparticles dispersed throughout. These nanoparticles combine hard magnetic MnBi and provide a nanoparticle material suitable for the production of a permanent magnet, which is a rare earth element-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 obtained from nanoparticulate materials obtained from wet-chemical synthesis processes. For example, the US application filed on September 12, 2013, with the disclosure US 2015 / 0 068 646 A1, discloses MnBi nanoparticles with a particle size of 5 to 200 nm as a source of hard magnetic materials. Furthermore, the application filed on April 14, 2014, with the disclosure US 2015 / 0 294 775 A1, discloses core-shell nanoparticles comprising an iron-cobalt nanoparticle core of less than 200 nm with a silicon dioxide shell and a metal-silicate interface, as a source of soft magnetic materials.

[0003] Magnetic materials can generally be divided into two classes: hard magnetic materials, which can be permanently magnetized, 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 preferentially avoided. High core losses are therefore characteristic of permanent magnetic materials but undesirable in soft magnetic materials.

[0004] Many of today's advancing technologies require an efficient and strong hard magnet as a fundamental component of device construction. Such devices range from mobile phones to high-performance electric motors, and significant efforts are ongoing across the board to find materials that not only meet current requirements but also meet the ever-growing demand for efficient, more cost-effective, and easily manufactured hard magnetic materials.

[0005] Traditionally, neodymium iron borate is generally considered one of the strongest and most powerful hard magnetic materials available. However, because this material is based on the rare earth element neodymium, it is expensive, and the available supply is often unstable. Accordingly, there is a need for a material that performs as well as or better than neodymium iron borate as a hard magnet, yet is based on materials with readily available and more cost-effective components.

[0006] Magnetic device components are manufactured from powders by compacting the powders into a defined shape and then sintering the compact at temperatures of 200°C or more. Sintering of the component following compaction is necessary to achieve satisfactory mechanical properties by ensuring interparticle bonds and, consequently, strength.

[0007] The publications US 2012 / 0 208 026 A1, US 2004 / 0 134 565 A1 and US 2014 / 0 027 667 A1 disclose magnetic core-shell nanoparticles from the state of the art.

[0008] Technological advances in all areas of the communications and power generation sectors require increasingly high-performance magnetic powders with controllable or adjustable magnetic properties, allowing the production of customized, low-cost, and readily available magnetic components.

[0009] One object of the present invention is therefore to provide a hard magnetic powder with a high coercive field strength for the production of permanent magnetic components. The powder must be inexpensive compared to conventional permanent magnet materials and readily available. Summary of the invention

[0010] These and other objects have been achieved according to the present invention, the first embodiment of which includes a core-shell-core nanoparticle comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate shell made of a silicon dioxide coating the superparamagnetic core; another nanoparticle core made of a manganese-bismuth alloy on the silicon dioxide intermediate shell; and a metal silicate interfacial layer between the superparamagnetic core and the silicon dioxide shell; wherein a diameter of the iron-cobalt alloy core is 200 nm or less.

[0011] In one embodiment, the diameter of the outer MnBi core may be from 0.5 to 200 nm.

[0012] In one aspect of the above two embodiments, the metal silicate interface may have a thickness of 0.5 to 10 nm, and the thickness may be controlled by the length of time of the wet synthesis to prepare the silica shell.

[0013] The foregoing paragraphs are provided as a general introduction and are not intended to limit the scope of the following claims. The presently preferred embodiments, together with further advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings. Short description of the drawings Fig. Figure 1 shows a TEM image of the core-shell-core nanoparticles prepared in the example. Fig. Figure 2 shows a scan of DSC and M(T) data for the core-shell-core nanoparticles prepared in the example. Fig. Figure 3 shows a Z-contrast TEM image of the core-shell-core nanoparticles prepared in the example. Detailed description of the invention

[0014] Unless otherwise stated, all ranges described in this description include all values ​​and subranges therein.

[0015] In addition, the indefinite article “a” or “an” has the meaning “one or more” throughout this description, unless otherwise stated.

[0016] In an ongoing investigation into 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 for the production of permanent magnets. MnBi nanoparticles have been predicted to exhibit high coercive field strengths of up to 4 T. The invention disclosed in the US application filed on September 12, 2013, with the publication number US 2015 / 0 068 646 A1, discloses some results of that work.

[0017] The inventors are also currently conducting ongoing investigations with soft magnetic nanoparticulate materials, such as those in the application filed on April 14, 2014, with the disclosure US 2015 / 0 294 775 A1, which discloses core-shell nanoparticles comprising an iron-cobalt nanoparticle core of less than 200 nm with a silicon dioxide shell and a metal-silicate interface.

[0018] In ongoing research with these and other systems, the inventors have surprisingly discovered that core-shell-core nanoparticles, obtained by applying a manganese-bismuth nanocoating to a core-shell nanoparticle containing a 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-shell-core components. Such a complex combination of soft and hard magnetic components within a nanoparticle is novel and offers many opportunities for the discovery and development of new magnetic materials and devices.

[0019] In a first embodiment, the present invention includes core-shell-core nanoparticles comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate sheath made of silicon dioxide coating the superparamagnetic core; an outer manganese-bismuth alloy nanoparticle, also referred to as an outer core based on the spherical nanoscale nature of the MnBi nanoparticle on the silicon dioxide interlayer; and a metal silicate interfacial layer between the superparamagnetic core and the silicon dioxide shell; wherein a diameter of the iron-cobalt alloy core is 200 nm or less.

[0020] The inventors discovered that the formation of individual FeCo alloy nanoparticles coated with silica shells of varying thicknesses can be achieved via a scalable wet chemical process. Surprisingly, the inventors discovered that the formation of interfacial metallosilicates can significantly alter the nanomagnetism in these ultrahigh surface area FeCo alloy nanoparticle systems. Evidence that an interfacial metallosilicate layer had formed was observed in X-ray photoelectron spectra acquired across the Fe and Co 2p transitions; and as the thickness of the silica shell was increased (by altering the duration of the silica reaction), a thicker interfacial metallosilicate layer was formed, thus increasing the overall magnetic anisotropy of the nanoparticles, as evidenced by increased blocking temperatures and altered coercivities.The inventors have thus surprisingly discovered that by producing superparamagnetic iron-cobalt alloy nanoparticles encapsulated in silicon dioxide shells with varying wet synthesis treatment times, 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 further 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 particle of the iron-cobalt alloy and are thus superparamagnetic. Although not limited by theory, the inventors believe that controlling the grain size to approximately match that of the magnetic domain of the particles is a factor contributing to the reduced hysteresis of a magnetic core according to the present invention. Furthermore, the presence of insulating silica shells around the core grains is a factor contributing to the low generation of eddy currents of 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 simultaneously formed during the synthesis of the silicon dioxide shell. Evidence that an interfacial layer of metal silicates had formed was found in X-ray photoelectron spectra obtained from the Fe and Co 2p transitions; and as the thickness of the silicon dioxide shell was increased (by varying the duration of the silicon dioxide reaction), a thicker interfacial metal silicate layer was formed, thus increasing the overall magnetic anisotropy of the nanoparticles, as evidenced by increased blocking temperatures and altered coercivities. The inventors recognized that understanding the effect of this interfacial metal silicate layer on controlling magnetic properties is a critical element for effectively utilizing these materials in applications such as low-loss transformer cores.

[0024] In a study of FeCo alloy core-shell nanoparticles, the inventors discovered that interfacial metal silicates formed during the synthesis of the silica shell coating alter the overall magnetic anisotropy of the nanoparticles as a higher anisotropy phase, 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 bare FeCo nanoparticle.

[0025] Samples of binary FeCo alloy nanoparticles with a single magnetic domain were synthesized (see example), except that the SiO2 reaction times were varied, resulting in SiO2 shells of varying thicknesses: a reaction time of 1 min resulted in a 3 nm thick shell, 10 min resulted in a 4 nm thick shell, and 20 min resulted in a 6 nm thick shell. The average diameter of the FeCo nanoparticles and the thickness of the SiO2 shell were determined, and for all three core / shell nanoparticle samples (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 shells was determined in a similar manner and yielded 3 ± 1 nm, 4 ± 1 nm, and 6 ± 1 nm for the FeCo / SiO2(3 nm), FeCo / SiO2(4 nm), and FeCo / SiO2(6 nm) samples, respectively.The TEM images revealed that the FeCo cores were completely covered by the silica shells. Analysis of the X-ray diffraction patterns indicated the presence of both Fe and Co silicates. However, the relative proportions appear to fluctuate, and the inventors hypothesize, without wishing to be limited to theory, that the metal silicate content may be related to the thermodynamic energy of metal silicate formation. The investigations showed that Fe and Co silicates formed at the interface between the FeCo nanoparticle core and the SiO2 shell during the synthesis process. However, the relative integrated areas of the FeO and CoO metal peaks of the various core / shell 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 the base catalyst to form silica shells. These particles can then be purified using an aqueous ethanol rinse.

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

[0028] The synthesis may 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] The manganese-bismuth alloy coating can be formed by a process comprising, in the presence of the FeCo-silica core-shell nanoparticles: treating a Mn powder with a hydride reducing agent and blending by ball milling; adding, with continued stirring, a solution of a bismuth salt of a long-chain carboxylate and an alkylamine to the Mn-hydride reducing agent mixture; after completion of the addition of the bismuth salt solution, continuing stirring to form the core-shell-core FeCo / SiO2 / MnBi nanoparticles.

[0030] The ether solvent for the hydride treatment can be any ether compatible with hydride reaction conditions. Suitable ether solvents include 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.

[0031] The hydride reducing agent can be any material capable of reacting with the manganese to form a manganese-reducing agent complex, including NaH, LiH, CaH2, LiAlH4, and LiBH4. LiBH4 may be a preferred hydride treatment agent.

[0032] The formation of a manganese-lithium borohydride reducing agent complex is achieved by milling the manganese powder and hydride reducing agent in a planetary ball mill at 150 to 400 rpm for up to four hours. Variations of this procedure can be optimized to appropriately modify the resulting properties and would be understood by one of ordinary skill in the art.

[0033] In addition, the amount of the hydride treatment agent can be varied to modify conditions and the properties of the resulting nanoparticles and can vary in an equivalent ratio of hydride to Mn from 1 / 1 to 100 / 1.

[0034] 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 Bi is added as bismuth neodecanoate. The molar ratio of Bi to Mn can vary from 0.8 / 1 to 1.2 / 1. Preferably, the Bi / Mn ratio is from 0.9 / 1 to 1.1 / 1, and most preferably, the Bi / Mn ratio is 1 / 1. The addition time of the bismuth compound can be varied to optimize and modify the size and properties of the MnBi. The width can be from 0.5 to 200 nm, preferably 1.0 to 100 nm, and most preferably 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.

[0035] The alkylamine is preferably a primary amine having a carbon chain of 6 to 12 carbons and can be added to the reaction if desired.

[0036] As in Fig. 2, both the soft-phase FeCo and the hard-phase MnBi anneal at temperatures characteristic of FeCo and MnBi, respectively, when the core-shell-core nanoparticles of the invention are thermally treated in an annealing process.

[0037] Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. EXAMPLE

[0038] Core-shell-core iron-cobalt / silicon dioxide / manganese-bismuth nanoparticles

[0039] 0.489 g of sodium hydroxide, 12.892 g of tetraoctylammonium bromide, 10.922 g of iron dichloride tetrahydrate, and 12.042 g of cobalt chloride hexahydrate were dissolved in 250 mL of ethanol and placed under argon. A solution of 2.258 g of sodium borohydride dissolved in 450 mL of 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 of water. The product, the FeCo nanoparticles, was then washed with 70% water / 30% ethanol.

[0040] The FeCo nanoparticles were then suspended in a mixture of 625 ml of water and 2 ml of triethylamine. A solution of 0.5 ml of tetraethyl orthosilicate in 390 ml of ethanol was then added to the FeCo suspension, and the resulting mixture was allowed to react for 15 minutes to obtain silica-coated nanoparticles. The coated nanoparticles were then washed with ethanol.

[0041] The silica-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.

[0042] A TEM image of the prepared core-shell-core nanoparticles is shown in Fig. 1 shown.

[0043] The Z-contrast TEM image of the Fig. Figure 3 shows how the MnBi phase exhibits an island distribution throughout the FeCo / SiO2.

[0044] Fig.Figure 2 shows DSC and M(T) data over the temperature ranges where the observed properties from both data sets indicate the clear presence of the soft magnetic FeCo phase and the hard magnetic MnBi phase, thus confirming the presence of these two in the core-shell-core nanostructure. 1 emu = 10 ↑-3 ↑ J / T.

Claims

[1] Core-shell-core nanoparticle system comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate sheath made of silicon dioxide coating the superparamagnetic core; an outer core of a manganese-bismuth alloy on the silicon dioxide intermediate cladding; and a metal silicate interface layer between the superparamagnetic core and the silicon dioxide shell; where a diameter of the iron-cobalt alloy core is 200 nm or less. [2] The core-shell-core nanoparticle system according to claim 1, wherein a diameter of the outer MnBi core is from 0.5 to 200 nm. [3] Core-shell-core nanoparticle system according to claim 1, wherein the metal silicate of the interfacial layer comprises iron silicate and / or cobalt silicate. [4] The core-shell-core nanoparticle system according to claim 1, wherein the thickness of the metal silicate interfacial layer is from 0.5 nm to 15 nm. [5] The core-shell-core nanoparticle system according to claim 1, wherein the superparamagnetic core is made of an iron-cobalt alloy. [6] The core-shell-core nanoparticle system according to claim 1, wherein the diameter of the iron-cobalt core is from 2 to 75 nm. [7] A process for preparing a core-shell-core FeCo / SiO2 / MnBi nanoparticle system comprising: Joint reduction of an iron ion and a cobalt ion from a common solution; and Coprecipitation of a FeCo alloy nanoparticle; Isolating the FeCo nanoparticle from the reduction mixture; Forming a silica coating on the FeCo nanoparticle to obtain a core-shell nanoparticle, wherein the obtained nanoparticles are treated with tetraethyl orthosilicate in a water-ethanol mixture using triethylamine as a base catalyst to form silica shells; and Forming a MnBi alloy nanoparticle on the core-shell nanoparticle by reducing Bi ions with ions of a Mn and lithium borohydride reducing agent complex to obtain a MnBi alloy nanoparticle on the silicon dioxide shell. [8] Core-shell-core nanoaggregation of FeCo / SiO2 nanoparticles with MnBi nanoparticles attached to the surface of the silica shell.

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