Superparamagnetic iron cobalt ternary alloy and silica nanoparticles of high magnetic saturation and a magnetic core containing the nanoparticles

Abstract

Thermally annealed superparamagnetic core shell nanoparticles of an iron-cobalt ternary alloy core and a silicon dioxide shell having high magnetic saturation are provided. A magnetic core of high magnetic moment obtained by compression sintering the thermally annealed superparamagnetic core shell nanoparticles is also provided. The magnetic core has little core loss due to hysteresis or eddy current flow.

Claims

1. A magnetic core, comprising: superparamagnetic grains of an iron cobalt ternary alloy; and a matrix of silicon dioxide as a shell on to the superparamagnetic grains; wherein a diameter of the iron cobalt ternary alloy grain is from 3 to 35 nm, the third component of the ternary alloy is a transition metal selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, nickel, copper and zinc, the magnetic core is superparamagnetic, and the magnetic core is a monolithic structure obtained by a process comprising: wet chemical precipitation of the iron cobalt alloy grain; coating of the grain with a silicon dioxide shell to obtain a thermally untreated core shell nanoparticle having a magnetic saturation (M.sub.s); and thermal annealing of the untreated core shell nanoparticle to obtain the thermally, annealed superparamagnetic core shell nanoparticle having a magnetic saturation (.sup.TAM.sub.s); wherein .sup.TAM.sub.s is equal to or greater than 1.25 M.sub.s and sintering the thermally annealed core shell nanoparticles under pressure to form the monolithic structure of thermally annealed superparamagnetic core grains of an iron cobalt ternary alloy directly bonded by the silicon dioxide shells, which form a matrix.

2. The magnetic core according to claim 1, wherein the thermal annealing comprises heating the core shell nanoparticle having a magnetic saturation (M.sub.s) at a temperature of from 150° C. to 600° C. for from 3 to 180 seconds.

3. The magnetic core according to claim 1, wherein a coercivity value of the thermally untreated core shell nanoparticle (H.sub.C) and a coercivity value of the thermally treated core shell nanoparticle (.sup.TAH.sub.C) are substantially equal.

4. The magnetic core according to claim 1, wherein a space between individual thermally annealed superparamagnetic nano iron cobalt ternary alloy grains is occupied substantially only by the silicon dioxide.

5. The magnetic core according to claim 1, wherein the thermally annealed superparamagnetic core comprises an iron cobalt vanadium alloy.

6. The magnetic core according to claim 1, wherein the thermally annealed superparamagnetic core consists of an iron cobalt vanadium alloy.

7. The magnetic core according to claim 1, wherein at least 97% by volume: of the space between the thermally annealed superparamagnetic core grains of iron cobalt ternary alloy is occupied by silicon dioxide.

8. The magnetic core according to claim 1, wherein the monolithic core comprises no binder and no resin.

9. An electrical/magnetic conversion device, which comprises the magnetic core according to claim 1.

10. A vehicle part comprising the electrical/magnetic conversion device according to claim 9, wherein the part is selected from the group consisting of a motor, a generator, a transformer, an inductor and an alternator.

11. A method to prepare the magnetic core of claim 1, comprising: wet chemical precipitation of the iron cobalt alloy grain; coating of the grain with a silicon dioxide shell to obtain a thermally untreated core shell nanoparticle having a magnetic saturation (M.sub.s) and thermal annealing of the untreated core shell nanoparticle to obtain the thermally annealed superparamagnetic core shell nanoparticle having a magnetic saturation (.sup.TAM.sub.s); wherein .sup.TAM.sub.s is equal to or greater than 1.25 M.sub.s; and sintering the thermally annealed superparamagnetic core shell nanoparticles under heat and pressure under flow of an inert gas to obtain the monolithic structure.

12. The method according to claim 11, wherein the thermal annealment comprises heating the core shell nanoparticles at a temperature of from 150° C. to 600° C. for from 3 to 180 seconds.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the hysteresis curves for a sample annealed according to an embodiment of the invention in comparison to the material before annealing.

(2) FIG. 2 shows a generalized relationship of particle size and range of superparamagnetism.

DETAILED DESCRIPTION OF THE INVENTION

(3) The inventors have discovered that a thermal annealing treatment of the superparamagnetic core shell nanoparticles following preparation of the core shell structure results in the production of a magnetic material having markedly different magnetic properties in comparison to the similarly prepared materials which are not annealed. Thus the inventors have surprisingly discovered that by producing superparamagnetic iron cobalt ternary alloy nanoparticles that are encapsulated in silica shells, thermally annealing the nanoparticles under specific conditions related to the particle size and composition and then compacting and sintering these nanoparticles into a monolithic nanomaterial core, the core obtained, in addition to having zero (or very low) hysteresis and very low eddy current formation has a high magnetic moment.

(4) Thus, the first embodiment of the present invention provides a thermally annealed superparamagnetic core shell nanoparticle, comprising: a superparamagnetic core of an iron cobalt ternary alloy; and a shell of a silicon oxide directly coating the core; wherein a diameter of the iron cobalt ternary alloy core is 200 nm or less, preferably 50 nm or less, more preferably 3 to 35 nm and most preferably 5 to 15 nm. The core shell particle may be obtained by a process comprising: wet chemical precipitation of the core nanoparticle; coating of the core nanoparticle with a silicon dioxide shell to obtain a thermally untreated core shell nanoparticle having a magnetic saturation (M.sub.s); and thermal annealing of the untreated core shell nanoparticle to obtain the thermally annealed superparamagnetic core shell nanoparticle having a magnetic saturation (.sup.TAM.sub.s); wherein .sup.TAM.sub.s is equal to or greater than 1.25 M.sub.s.

(5) The third component of the iron cobalt ternary alloy is selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, nickel, copper and zinc.

(6) In a highly preferred embodiment, according to the present invention, the iron cobalt ternary alloy is an iron cobalt vanadium alloy.

(7) In another highly preferred embodiment according to the present invention, the iron cobalt ternary alloy is an iron cobalt chromium alloy.

(8) According to the invention, the iron cobalt ternary alloy nanoparticle grains are of or approaching the size of the single particle magnetic domain of the iron cobalt ternary alloy and thus are superparamagnetic. While not being constrained to theory, the inventors believe control of grain size to approximately that of the particle magnetic domain is a factor which contributes to the reduced hysteresis of a magnetic core according to the present invention. Moreover, the presence of insulating silica shells about the core grains is a factor which contributes to the low eddy current formation of a magnetic core according to the present invention.

(9) It is conventionally known that the range of particle size for which single domain particles exhibit superparamagnetism has an upper boundary characteristic of the particle chemical constitution. This phenomenon is shown in FIG. 2 which is reproduced from Nanomaterials An Introduction to Synthesis, Properties and Applications by Dieter Vollath (page 112) Wiley-VCH. According to FIG. 2, above a certain size range, nanoparticles will exhibit a measurement time dependency characteristic of ferromagnetic behavior. To avoid this time dependency nanoparticles of a size within the range of superparamagnetism must be prepared and that size maintained during further processing.

(10) The inventors have discovered that by a process of rapid thermal annealing of the iron cobalt ternary alloy core shell nanoparticles according to the invention the M.sub.S is increased without significantly increasing the magnetic coercivity (H.sub.c). Although not being limited by theory, the inventor believes that in the material composition of these annealed nanoparticles that exhibit an increased M.sub.s the magnetic moment of the total nanoparticles is organized and produces a markedly different magnetic material property in comparison to the as synthesized material.

(11) The inventors believe that thermal annealing of magnetic materials allows for the relaxation of trapped-in defects formed in synthesis and thus, an improvement in magnetic properties (i.e. M.sub.S). However, at increased temperatures two conflicting processes are occurring within the nanoparticles. On one hand, the alignment of the particle crystal structure leading to a more pure crystallinity takes place; while at the same time the nanoparticles are prone to coalesce and grow in crystal size. These two phenomena have opposite effect on the magnetic properties of the nanoparticle and therefore, the annealing procedure must be designed to maximize perfection of crystallinity while at the same time minimizing nanoparticle growth. Thus, as thermal annealing allows for the relaxation of crystal structures, it may also result in particle-particle growth despite the encapsulating silica shells. High specific surface area materials such as the superparamagnetic nanoparticles (SPNPs) according to the invention are especially prone to particle growth as they are thermodynamically-driven to reduce their surface energy. Such particle growth is particularly detrimental for application as a core material, since particles that are too large no longer exhibit superparamagnetic (single domain) properties, and will exhibit an unacceptably large H.sub.c.

(12) To avoid such particle growth the inventors have discovered that with iron cobalt ternary alloy nanoparticles, rapidly annealing the core/shell SPNPs kinetically limits the amount of particle growth.

(13) Nanoparticles of Fe—Co—V/SiO.sub.2 may be synthesized by the ethanolic reaction of sodium borohydride with iron dichloride, cobalt dichloride and vanadium trichloride in a solution of sodium hydroxide and tetraoctylammonium bromide. The obtained nanoparticles may be treated with tetraethyl orthosilicate, in water ethanol mixture using triethylamine as the base-catalyst, to form silica shells. These particles may then be purified using an aqueous ethanol rinse.

(14) Annealing temperatures may be varied between 150° C. and 600° C., while annealing times (at temperature) may be from 1 second to 3.5 minutes. In one embodiment the sample is heated at 200° C. for 20 seconds.

(15) A Quantum Design VersaLab™ vibrating sample magnetometer (VSM) may be used to obtain the M-H hysteresis curves for the nanopowders. VSM analysis may be conducted at 300 K in a low pressure (˜40 torr) atmosphere. The hysteresis curve for the sample annealed at 200° C. for 20 seconds is compared to the same material prior to annealing in FIG. 1.

(16) The inventor has discovered that actual optimal annealing time and temperatures may vary with lot to lot produced nanoparticles, depending on factors such as, for example, actual particle size, particle size distribution and chemical composition of the nanoparticles. Thus the optimum time at a given temperature for a given nanoparticle batch may be determined by the procedures described above.

(17) In general, for Fe—Co ternary alloy nanoparticles prepared as described above, annealing times of about 3 to 180 seconds, preferably, 10 to 50 seconds at annealing temperatures of about 180 to 550° C. are effective according to the invention. These values include all sub-ranges and specific temperatures and times within these ranges. In a preferred embodiment the time of annealing at 200° C. is from 15 to 30 seconds.

(18) Thus, as shown by the data in FIG. 1, magnetic saturation value may be increased by a factor of almost 3 times. Value increases of 15 to 58 emu/g may be obtained upon annealing according to the invention. Correspondingly, coercivity values were found to not change appreciably during annealing under the conditions according to the invention, indicating the particles remain in their single-domain nano-scale state.

(19) In another embodiment, the present invention includes a magnetic core, comprising: the thermally annealed iron-cobalt ternary alloy core shell nanoparticles having a particle size of less than 200 nm, preferably less than 50 nm; wherein the core is an iron-cobalt ternary alloy and the shell is a silicon oxide and the magnetic core is a monolithic structure of superparamagnetic core grains of iron-cobalt ternary alloy directly bonded by the silicon oxide shells. Preferably the particle size is from 3 to 35 nm and most preferably from 5 to 15 nm. These ranges include all subranges and values there between.

(20) The core according to the present invention is monolithic, having the space between the thermally annealed iron-cobalt ternary alloy nanoparticle grains occupied by the silicon oxide. Preferably at least 97% of the space between the grains, preferably 98% and most preferably 100% of the space is silicon oxide and further most preferably the silicon oxide is silicon dioxide. According to the present invention neither any binder nor any resin is contained in the matrix of the monolithic core.

(21) The monolithic core according to the present invention is obtained by a process comprising sintering a powder of the thermally annealed superparamagnetic core shell particles having a particle size of less than 200 nm under pressure under flow of an inert gas to obtain a monolithic structure; wherein the core of the core shell particle consists of superparamagnetic iron-cobalt ternary alloy, preferably iron-cobalt-vanadium, and the shell consists of silicon dioxide. Because a magnetic material is only superparamagnetic when the grain size is near or below the magnetic domain size, the nanoparticle core must be maintained as small as possible, or the sample will become ferromagnetic, and express magnetic hysteresis. Therefore, the most mild and gentle sintering conditions that still yield a monolithic sample that is robust enough to be machined into a toroid are desired, because more aggressive sintering conditions will promote unwanted grain growth and potentially, loss of superparamagnetic performance.

(22) The magnetic core as described herein may be employed as a component in an electrical/magnetic conversion device, as known to one of ordinary skill in the art. In particular the magnetic core according to the present invention may be a component of a vehicle part such as a motor, a generator, a transformer, an inductor and an alternator, where high magnetic moment is advantageous.

(23) 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. Skilled artisans will recognize the utility of the devices of the present invention as a battery as well as the general utility of the electrolyte system described herein.

EXAMPLE

Core/Shell Iron-Cobalt-Vanadium/Silica-Coated Nanoparticles

(24) The nanoparticles were synthesized as follows:

(25) Sodium borohydride (2.45 g) was dissolved in ethanol (90 mL). This sodium borohydride solution was added to a stirring solution of ethanol (105 mL) containing:

(26) sodium hydroxide (0.102 g), tetraoctyalammonium bromide (2.362 g), iron dichloride tetrahydrate (2.181 g), cobalt dichloride hexahydrate (2.410 g), and vanadium trichloride (0.0683 g).

(27) The reaction was allowed to stir for 10 minutes to insure full reaction had taken place.

(28) It was then washed with a solution of ethanol and water (30/70 by volume, respectively) to remove the reaction byproducts.

(29) The nanoparticles were dispersed in a solution of water (126 mL) and triethylamine (3.3 mL) This suspension was mixed thoroughly.

(30) Tetraethyl orthosilicate (0.200 mL) dissolved in ethanol (78 mL) was then added and allowed to react for 20 mins.

(31) The product was washed with again with the solution of ethanol and water (30/70) and then pure ethanol to remove any reaction byproducts.

(32) These nanoparticles were thermally annealed at 200° C. for 20 seconds, which produced a nanoparticle material that was still superparamagnetic, but with substantially elevated magnetic saturation. (FIG. 1)

(33) Finally, this improved superparamagnetic nanoparticle was hot press sintered to form a compacted nanocomposite which was then fabricated into a magnetic core for use in devices such as transformers and inductors.

(34) The product of the hot press sintering was a disc. The size of the disk is dependent upon the size of punch and die set used. As described here but not limiting the dimensions of those stated, discs were produced that were 9 mm in diameter and 2.5 mm thick. The disc was converted to a toroid through conventional machining techniques. The fabricated toroid was hand-wound with copper enameled wire to produce an inductor.