Hard / soft magnetic MnBi / SiO2 / FeCo nanoparticles

Abstract

Core-shell-core nanoparticles are provided, consisting of an iron-cobalt alloy core, a silicon dioxide shell, and a manganese-bismuth alloy core or nanoparticles on the surface of the silicon dioxide shell (FeCo / SiO2 / MnBi). Due to the nanometer-scale proximity of the hard magnetic manganese-bismuth to the soft magnetic iron-cobalt, these core-shell-core nanoparticles are alternative materials to rare-earth permanent magnets.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The present invention relates to a magnetic nanoaggregate of nanoparticles comprising an iron-cobalt core with a silicon dioxide intermediate shell and manganese-bismuth alloy nanoparticles dispersed throughout. These nanoparticles combine soft magnetic MnBi and provide a nanoparticle material suitable for the production of a permanent magnet, which is 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. US Application No. 14 / 025 033, 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, US Application No. 14 / 252 036, 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. The disclosures of both applications are incorporated herein by reference in their entirety.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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 cost-effective compared to conventional permanent magnet materials and readily available. Summary of the invention

[0009] These and other problems were solved according to the present invention, the first embodiment of which comprises a core-jacket-core nanoparticle comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate jacket of silicon dioxide coating the core; a further nanoparticle core of a manganese-bismuth alloy on the silicon dioxide intermediate jacket; and a metal silicate interface layer between the core and the silicon dioxide jacket; wherein the diameter of the iron-cobalt alloy core is 200 nm or less.

[0010] In one embodiment, the diameter of the MnBi core can range from 0.5 to 200 nm.

[0011] In one aspect of the two embodiments above, the metal silicate interface can have a thickness of 0.5 to 10 nm, and the thickness can be controlled by the length of the wet synthesis duration for the production of the silicon dioxide sheath.

[0012] 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

[0013] Fig. Figure 1 shows a TEM image of the core-mantle-core nanoparticles produced in the example.

[0014] Fig. Figure 2 shows a sampling of DDK and M(T) data for the core-jacket-core nanoparticles produced in the example.

[0015] Fig. Figure 3 shows a Z-contrast TEM image of the core-mantle-core nanoparticles produced in the example. Detailed description of the invention

[0016] Unless otherwise stated, all areas described in this description include all values ​​and sub-areas therein.

[0017] Additionally, the indefinite article “ein” or “eine” throughout the description means “one or more”, unless otherwise stated.

[0018] 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 US Application No. 14 / 025 033, filed on September 12, 2013, discloses some results of that work.

[0019] The inventors are also currently conducting ongoing investigations with soft magnetic nanoparticulate materials, such as those disclosed in US patent application 14 / 252 036, filed on April 14, 2014, which discloses core-jacket nanoparticles featuring an iron-cobalt nanoparticle core of less than 200 nm with a silicon dioxide jacket and a metal-silicate interface.

[0020] In ongoing research with these and other systems, the inventors have surprisingly discovered that core-cladding-core 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-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.

[0021] In a first embodiment, the present invention includes core-cladding-core nanoparticles comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate shell made of silicon dioxide coating the core; an outer manganese-bismuth alloy nanoparticle, which, based on the spherical nanoscale structure of the MnBi nanoparticle on the silicon dioxide intermediate shell, is also referred to as a core; and a metal silicate interface layer between the core and the silicon dioxide mantle; where the diameter of the iron-cobalt alloy core is 200 nm or less.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 core / sheath 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 jackets was determined in a similar way and yielded 3 ± 1 nm, 4 ± 1 nm and 6 ± 1 nm for the samples FeCo / SiO2(3 nm), FeCo / SiO2(4 nm) and FeCo / SiO2(6 nm), respectively.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, their 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 its 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. However, the relative integrated areas of the Fe₂O and Co₂ metal peaks of the various core / shell nanoparticle systems suggested that Fe silicates may be preferentially formed over Co silicates.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] The manganese-bismuth alloy coating can be formed by a process that, in the presence of the FeCo-silicon dioxide core-shell nanoparticles, comprises: treating a Mn powder with a hydride reducing agent and mixing by milling in a ball mill; adding, with continuous stirring, a solution of a bismuth salt, 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 the stirring to form the core-shell FeCo / SiO2 / MnBi nanoparticles.

[0032] 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.

[0033] 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.

[0034] The formation of a manganese-lithium borohydride reducing agent complex is achieved by grinding 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 modify the obtained properties appropriately and would be understood by a person skilled in the art.

[0035] Additionally, the amount of hydride treatment agent can be varied to modify conditions and the properties of the obtained nanoparticles, and can vary in an equivalent ratio of hydride to Mn from 1 / 1 to 100 / 1.

[0036] 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. 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.

[0037] The alkylamine is preferably a primary amine with a carbon chain of 6 to 12 carbons and can optionally be added to the reaction.

[0038] As in Fig. As stated in 2, both the soft-phase FeCo and the hard-phase MnBi glow at temperatures characteristic of FeCo and MnBi respectively when the core-shell-core nanoparticles of the invention are thermally treated in an annealing process.

[0039] 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 any limitation unless otherwise stated. EXAMPLE Core-Shell-Core-Iron-Cobalt / Silicon Dioxide / Manganese-Bismuth Nanoparticles

[0040] 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 2.258 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 70% water / 30% ethanol.

[0041] 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.

[0042] 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.

[0043] A TEM image of the fabricated core-mantle-core nanoparticles is in Fig. 1 shown.

[0044] The Z-contrast TEM image of the Fig. Figure 3 shows how the MnBi phase exhibits an island distribution throughout the entire FeCo / SiO2.

[0045] Fig.Figure 2 shows DDK and M(T) data over the TEMPERATURE ranges in which 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. QUOTES INCLUDED IN THE DESCRIPTION

[0046] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0047] US 14 / 252036 [0002, 0019]

Claims

[1] Core-mantle-core nanoparticle system comprising: a superparamagnetic core comprising an iron-cobalt alloy; an intermediate shell made of silicon dioxide coating the core; an outer core made of a manganese-bismuth alloy on the silicon dioxide intermediate shell; and a metal silicate interface layer between the core and the silicon dioxide mantle; wherein a diameter of the iron-cobalt alloy core is 200 nm or less. [2] Core-jacket-core nanoparticle system according to claim 1, wherein the diameter of the 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 interface layer comprises iron silicate and / or cobalt silicate. [4] Core-jacket-core nanoparticle system according to claim 1, wherein the thickness of the metal silicate interface layer is from 0.5 nm to 15 nm. [5] Core-jacket-core nanoparticle system according to claim 1, wherein the superparamagnetic core consists of an iron-cobalt alloy. [6] Core-shell-core nanoparticle system according to claim 1, wherein the diameter of the iron-cobalt core is from 2 to 75 nm. [7] Method for producing 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 Joint failures of a FeCo alloy nanoparticle; Isolating the FeCo nanoparticle from the reduction mixture; Forming a silicon dioxide coating on the FeCo nanoparticle to obtain a core-jacket nanoparticle; and Formation of a MnBi alloy nanoparticle on the core-jacket nanoparticle by reduction of Bi ions with ions of a Mn and lithium borohydride reducing agent complex to obtain a MnBi alloy nanoparticle on the silicon dioxide jacket. [8] Core-shell-core nanoaggregation of FeCo / SiO2 nanoparticles with MnBi nanoparticles attached to the surface of the silicon dioxide shell.