KR102564042B1 - High performance magnetorheological fluids having magnetic anisotropic particle and preparation method threreof - Google Patents

Abstract

The magnetorheological of the present invention compresses ferromagnetic particles into a plate shape, thereby obtaining a high yield stress of the suspension by using the magnetic shape anisotropy effect, which causes a rapid rise in the magnetic moment of the particles at low magnetic field strength, and at the same time, the stability resulting from the plate-shaped particles. Due to the increase, the sedimentation rate of the suspended particles can be remarkably reduced, thereby significantly improving the stability of the magnetorheological suspension. In addition, by attaching other ferromagnetic nanoparticles to the surface of the ferromagnetic particles, the properties of the magnetorheological by synergistic effect are further strengthened, and at the same time, the surface roughness is increased, thereby producing a magnetorheological with excellent suspension stability. Since the response speed of the magnetorheological fluid to the magnetic field is very fast and reversible at the level of milliseconds (10 ^-3 seconds), and the stability is greatly improved, various suspensions, clutches, dampers and earthquake-resistant devices using magnetorheological fluids, and electric control devices are included. It can be widely used in the development of shock mitigation devices, haptic devices, and surface cleaning devices. In addition, since most of these magnetorheological fluids are conductive metals, they can be used as electrorheological fluids that react strongly to an external electric field with a much lower content than magnetorheological fluids.

Description

High performance magnetorheological fluids having magnetic anisotropic particle and preparation method threreof

The present invention relates to magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of plate-shaped magnetic particles; And to a magnetorheological fluid, characterized in that the magnetic particles having magnetic anisotropy are dispersed in the fluid at 5 to 50% by volume.

More specifically, a magnetic particle plate-shaped process of forming plate-shaped magnetic particles using a fine forging process of magnetic particles; a nanoparticle attachment step of attaching ferromagnetic nanoparticles to the surface of the plate-shaped magnetic particles; and dispersing a magnetic fluid having magnetic anisotropy on the surface of which ferromagnetic nanoparticles are attached to the fluid.

Magnetorheological fluids are one of smart intelligent materials that can reversibly control viscosity in response to changes in magnetic field. Magnetorheological fluid is a suspension in which ferromagnetic and paramagnetic particles with a diameter greater than 0.1 μm are dispersed in an oil medium, and when an external magnetic field is applied, the particles are arranged by polarization on the inside and surface of the particles. A fibril structure is formed (see FIG. 1(b)), and the fibrous aggregate rapidly improves the viscosity of the suspension in a short time (within 10 milliseconds) to impede the flow of the fluid. Due to this increased viscosity, the suspension resists external stress like a solid, and when the external stress increases, flow occurs, and the stress at this time is called yield stress.

In general, the performance of a magnetorheological fluid is determined by the magnitude of the yield stress, and the yield stress increases with the strength of the magnetic field. When cut off, the suspension immediately returns to a liquid-like state from a fibrous coagulated state (see FIG. 1(a)). The response speed of magnetorheological fluid to this change in magnetic field is very fast at the level of milliseconds (10-3 seconds) and is reversible, so magnetorheological fluid belongs to smart materials, clutch, engine mount, vibration control device, high-rise building earthquake-proof device , robotic systems, suspensions, and biomedical engineering.

As a prior art related to this, as disclosed in Korean Patent Publication No. 10-2016-0011276, efficient In order to manufacture a magnetorheological fluid, above all, it is essential to manufacture a magnetorheological fluid having a high yield stress, and for this purpose, a method of increasing the volume fraction of magnetic particles or applying a strong magnetic field may be used. However, in general, when the volume fraction of magnetic particles is increased, the load and driving power consumption of the equipment are increased, and when a strong magnetic field is applied, the viscosity is increased when the magnetic field is not loaded, which is undesirable. Efforts are being made to develop magnetorheological fluids for this purpose and to apply them effectively.

In this regard, U.S. Patent Registration No. 2,575,360 as prior art exemplifies a torque converter that can be used in clutches and brakes, and magnetic particles (carbonyl iron) as a composition of magnetorheological fluid that can be used in this equipment are light lubricants. (light lubricant oil) is disclosed as a fluid dispersed in a volume fraction of 50%.

In addition, Korean Patent Laid-open Publication No. 10-2016-0011276 surrounds ferromagnetic particles with polymers to form multi-layer structured particles, and by undergoing a foaming process, the multi-layer structured particles have porosity, thereby reducing density, increasing stability, and settling speed was significantly reduced. However, in this case, the distance between the particles is increased by surrounding the magnetic particles with a polymer, which is a non-magnetic material, and there is also a disadvantage in that the yield stress of the magnetorheological is lowered.

On the other hand, one of the important factors in the actual application of magnetorheological fluids is that magnetorheological behavior is greatly affected by the sedimentation of particles by gravity. The main cause of this precipitation is due to the difference in density between the ferromagnetic particles (density of 7.86g/cm ³ ) and the continuous phase (density of 0.95g/cm ³ in the case of silicon oil), which leads to a decrease in the stability of the magnetorheological fluid. In order to overcome these disadvantages, US Patent Registration No. 5,043,070 attempted to stabilize the magnetorheological fluid using magnetic particles coated with two layers of surfactants, but the effect did not reach a satisfactory level.

In addition, in US Patent Publication No. 5,645,752, an attempt was made to minimize precipitation of magnetic particles by mixing a shear thinning additive with magnetorheological fluid to induce thixotropic crosslinking (network) for hydrogen bonding, but also a significant increase in stability failed to show

Recently, the Pickering emulsion process, which has emerged as an effective method for manufacturing nuclear envelope structure particles (Langmuir 2018, 34, 2807-2814.), is actively researched as a new magnetorheological particle manufacturing technique as a method for using solid particles as a surfactant. It became. Using the fact that nanometer-sized solid particles are located at the interface to reduce interface energy, when nanomagnetic particles are added during emulsion polymerization, core (polymer)-shell (solid surfactant) particles are obtained, and the core is a polymer Since these core (polymer)-shell (magnetic particles) particles have a low density, they reduce the density mismatch of the suspension and improve stability. However, since the ratio of magnetic particles present on the surface of the polymer core is extremely small, the magnetorheological property of the suspension is inferior to that of pure magnetic particle suspension (that is, shows a lower yield stress). Therefore, the need to develop a magnetorheological fluid with improved stability and high performance has been constantly emerging.

Republic of Korea Patent Publication No. 10-2016-0011276. U.S. Patent Registration No. 2,575,360 U.S. Patent Registration No. 5,043,070 U.S. Patent Registration No. 5,645,752

S. Han, J. Choi, Y. P. Seo, I. Park, H. J. Choi, Y. Seo, Langmuir 2018, 34, 2807-2814.

The present invention was developed to solve the above problems, and an object of the present invention is to provide a magnetorheological fluid having a high yield stress without increasing the volume fraction of magnetic particles.

In addition, it is intended to provide a high-performance magnetorheological fluid with improved stability that improves the stability degradation of the magnetorheological fluid due to the sedimentation problem of particles due to gravity.

In addition, it is intended to provide an electrorheological fluid that reacts under an electric field.

In the present invention, in order to solve the above problems, magnetic particles are made into a plate shape through a micro forging process, and the shape anisotropy effect (magnetic property dependence on the direction of the applied magnetic field) of the plate structure at this time is used. This effect is also called magnetic anisotropy and is different from intrinsic crystalline anisotropy, which describes the tendency of magnetization to align along a preferred crystallographic direction. Depending on the processing method, shape anisotropy can be more easily magnetized in an axial direction in which magnetization is easier in the case of anisotropic particles (a more deformed direction: a direction parallel to the plane direction in the case of plate-shaped particles). Then, by using this shape anisotropy effect, the magnetorheological performance of the magnetic material can be further improved even with a low magnetic field.

In addition, since the coefficient of frictional resistance of the particles depends on both the shape and the surface roughness, the stability of the suspension is improved by using the geometrical shape of the particles. Most previous studies on magnetorheological fluids have focused on developing synthetic routes for magnetic particles to improve the long-term stability of fluids, in contrast, research forms focusing on particle shape have been less developed. This is because there are not many non-spherical particles of the appropriate size, i.e. small enough to prevent settling, but sufficiently large to achieve a magnetic force sufficient to overcome thermal motion. That is, in the present invention, it is intended to prepare a more stable magnetorheological suspension while maintaining high performance by using the anisotropy of the particle shape and the magnetic shape.

In addition, in the present invention, by attaching other ferromagnetic particles manufactured in a micrometer size or smaller nanometer size to the surface of the shape anisotropic particle described above, the performance is improved by using the synergistic effect between the ferromagnetic particles, and at the same time, the increase in surface roughness It is intended to provide a magnetorheological suspension in which magnetorheological performance and stability are simultaneously improved by increasing the sedimentation stability of suspension particles by increasing fluid resistance.

In addition, in the present invention, since most of the high-performance magnetorheological fluid particles are conductive particles, the content is much smaller and mixed with other insulating particles within a range in which an electrical short does not occur to provide an electric fluid that reacts under an electric field.

The shape anisotropic particles prepared according to the present invention induce a rapid increase in the particle magnetic moment in the long axis direction (in the plane direction of the plate-like particles) under a low magnetic field due to a low demagnetization factor, thereby causing a high yield stress of the suspension. .

The shape anisotropic particles according to the present invention are more easily magnetized along the longitudinal direction (planar direction of the plate-like particles) and can form strong fibrous aggregates under a weak magnetic field, showing a practical method for producing a high-performance magnetorheological fluid.

In addition, by attaching other ferromagnetic particles of much smaller size (micrometer or less) to the surface of these shape anisotropic particles, the performance of the magnetorheological due to the synergistic effect between two different particles is improved and at the same time the large coefficient of frictional resistance of the plate-shaped particles Significantly improved stability is obtained by reducing the settling rate using

These results show that the demagnetization factor derived from the shape anisotropy of the magnetorheological fluid for a specific application field plays an important role and can be widely used in the development of various equipment using high-performance magnetorheological.

In addition, since most of these high-performance magnetorheological fluid particles are conductive particles, they can be applied as an electric fluid that reacts under an electric field by mixing with other insulating particles within a range that does not cause an electrical short, by making the content much smaller.

1 is an optical microscope image of a microstructure change of a bulk Sendust (BS) suspension according to an embodiment of the present invention, showing (a) before application of an external magnetic field and (b) after application of an external magnetic field.
Figure 2 is a scanning electron micrograph showing the shape of particles according to an embodiment of the present invention (a) carbonyl iron (carbonyl iron, CI), (b) bulk sendust (bulk Sendust, BS) and (c) plate-shaped It represents flake Sendust (FS).
Figure 3 is a scanning electron microscope image of the surface of the shape anisotropic particle to which other ferromagnetic nanoparticles are attached to the surface according to an embodiment of the present invention (a) pure FS (flake sandust), (b) synthesized Co _0.4 Fe _0.4 Ni _0.2 particles, ⓒ FS@Co _0.4 Fe _0.4 Ni _0.2 , and (d) shows an enlargement of the surface of (c).
Figure 4 shows (a) vibrating sample magnetometer (VSM) data according to an embodiment of the present invention: pure CI (7.8 g/cm ³ ), BS (7.0 g/cm ³ ), FS (7.2g) /cm ³ )particles (1kOe =10 ³ /(4π) kA/m), (b) Co _0.4 Fe _0.4 Ni _0.2 ,FS, and FS@Co _0.4 Fe _0.4 Ni _0.2 VSM measurement results are shown.
Figure 5 shows the magnetorheological characteristics of a magnetorheological body according to an embodiment of the present invention, (a) shear stress of CI, BS, FS suspensions for 10 vol% MR fluid at different magnetic field strengths, (b) shear viscosity, (c) Shear stress as a function of shear rate and (d) viscosity curves for suspensions of FS and FS@Co _0.4 Fe _0.4 Ni _0.2 .
Figure 6 is the yield stress values of magnetorheological suspensions experimentally measured using a rheometer according to an embodiment of the present invention (a) CI, BS and FS of 10 vol% at magnetic field strengths of 86 kA / m and 343 kA / m Shear viscosity versus shear stress for magnetorheological. (b) Shear viscosity as a function of shear stress for FS and FS@Co _0.4 Fe _0.4 Ni _0.2 MR fluids.
Figure 7 is an experimental result showing the stability of suspensions according to an embodiment of the present invention (a) transmittance (%) curve as a function of time [h] for CI, bulk sandust and flake sandust for 24 hours. (b) Transmittance (%) curves as a function of time (h) for Co _0.4 Fe _0.4 Ni _0.2 , FS and FS@Co _0.4 Fe _0.4 Ni _0.2 particles for 24 hours.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims should not be construed as being limited to ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to explain their invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The shape anisotropy effect, a term used throughout this specification, exploits the dependence of magnetic properties on the direction of an applied magnetic field. It is a different phenomenon from intrinsic crystalline anisotropy. Depending on the processing method, the shape anisotropy can be more easily magnetized in the axial direction in which magnetization is easier in the case of anisotropic particles (more deformed direction: in the case of plate-shaped particles, the direction parallel to the plane direction). Then, if this shape anisotropy effect is used, it means that the magnetorheological performance of the magnetic material can be further improved even with a low magnetic field.

In order to solve the above problems, the present invention includes a magnetic particle plate-shaped process of forming plate-shaped magnetic particles by using a fine forging process of magnetic particles; and a nanoparticle attachment step of attaching ferromagnetic nanoparticles to the surface of the plate-shaped magnetic particles.

According to one embodiment of the present invention, the nanoparticle attachment process may be to attach the ferromagnetic nanoparticles to the surface of the planar magnetic particles through a reduction reaction of a metal salt mixture, which is a precursor of the plate-like magnetic particles and the ferromagnetic nanoparticles, in the presence of a solvent, , The metal salt, which is the precursor of the ferromagnetic nanoparticles, may be a metal salt of nickel, cobalt, and iron, more preferably, the nickel salt is CoCl2 6H2O, the iron salt is FeCl2 4H2O, and the nickel salt is Ni(NO3)2 6H2O. can

According to one embodiment of the present invention, the magnetic particles are selected from the group consisting of iron, carbonyl iron, iron alloys, iron oxide, iron nitride, carbide iron, low carbon steel, nickel, cobalt, mixtures thereof, and alloys thereof It may be one or more, and the ferromagnetic nanoparticles are one selected from the group consisting of iron, carbonyl iron, iron alloys, iron oxide, iron nitride, carbide iron, low carbon steel, nickel, cobalt, mixtures thereof, and alloys thereof. It may be ferromagnetic particles of nanometer size or more.

In addition, the present invention provides magnetic particles having magnetic anisotropy, produced according to the above method, characterized in that ferromagnetic nanoparticles are attached to the surface of the plate-shaped magnetic particles.

In addition, the present invention includes a magnetic particle manufacturing process of manufacturing magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of plate-shaped magnetic particles according to the above method; And it provides a method for producing a magnetorheological fluid comprising the step of dispersing the magnetic particles having magnetic anisotropy in the fluid.

According to one embodiment of the present invention, the fluid may be selected from the group consisting of lubricating oil, mineral oil, silicone oil, castor oil, paraffin oil, vacuum oil, cone oil, and hydrocarbon oil, and the magnetic particles having magnetic anisotropy. is preferably dispersed in the fluid at 5 to 50% by volume.

In addition, in the present invention, magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of plate-shaped magnetic particles; and a fluid, wherein the magnetic particles having magnetic anisotropy are dispersed in the fluid at 5 to 50% by volume.

According to one embodiment of the present invention, the plate-like magnetic particles and the ferromagnetic nanoparticles attached to the surface are iron, carbonyl iron, iron alloys, iron oxide, iron nitride, carbide iron, low carbon steel, nickel, cobalt, mixtures thereof, and At least one selected from the group consisting of alloys thereof, and the fluid may be selected from the group consisting of lubricating oil, mineral oil, silicone oil, castor oil, paraffin oil, vacuum oil, cone oil, and hydrocarbon oil.

In addition, the magnetorheological fluid in the present invention can be used for suppressing or controlling shock or vibration of suspensions, clutches, shock mitigation devices, vibration control devices, earthquake-resistant devices, and polishing devices.

In addition, magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of the plate-shaped magnetic particles; It provides an electrorheological fluid comprising heat transfer particles and a fluid, wherein the magnetic particles having magnetic anisotropy are dispersed in the fluid at 5 to 50% by volume. At this time, since most of the high-performance magnetorheological fluid particles are conductive particles, they can be applied as an electric fluid that reacts under an electric field by mixing with other insulator particles within a range that does not cause an electrical short with a much smaller content.

Hereinafter, implementation examples and embodiments of the present invention will be described in detail so that those with general knowledge in the technical field to which the present application pertains can easily practice, as shown in the accompanying drawings, preferred embodiments of the present invention. In particular, the technical idea of the present invention and its core configuration and operation are not limited by this. In addition, the contents of the present invention can be implemented in various types of equipment, and is not limited to the implementation examples and examples described herein.

Method for producing magnetorheological fluid according to an embodiment of the present invention is Fe-Si-Al alloy powder, Fe-Ni alloy powder and Ni-Fe-Mo alloy powder, Fe-Si alloy powder, Fe-Co alloy powder, Fe -Cr-Si alloy powder, Fe-Al alloy powder, Fe-Cr alloy powder, and magnetic powder containing one or more types selected from magnetic powder such as Fe-Co-Ni, etc. After making it, using it as it is, or attaching other ferromagnetic particles synthesized at a level of several microns to less than a micron to the surface of the plate-shaped particles.

Plate-shaped magnetic particles according to an embodiment of the present invention can be made by micro-milling or micro-forging (micro forging) the magnetic metal powder for a predetermined period of time. The milling method is not limited to any specific process, and ball milling, attrition milling, bead milling, ultrasonic milling, and the like are possible. In order to enable uniform processing of the powder, wet milling with the application of a dispersant is preferred, but any other milling process may be used. Preferably, it can be obtained by wet attrition milling of metal powder by applying a dispersant such as xylene, toluene, ethanol, methanol, methyl ethyl ketone and cyclohexane, but for convenience of the process, a ball milling process or a microforging process, etc. This is easier, and the size of the micro balls used at this time, the input ratio of the milling balls to the powder, the milling time, etc. are process condition variables that can be changed according to the optimal conditions for plate-shaped production of each particle, but are not limited to any specific conditions. The plate-shaped particles produced through the milling process may be dried under a nitrogen atmosphere for a predetermined period of time to remove the dispersant and moisture used in the milling process, or may undergo a low-temperature aging step to remove residual stress.

Hereinafter, the manufacturing method of the magnetorheological fluid of the present invention will be described in more detail. However, the following examples are only examples of the present invention and the present invention is not limited thereto.

<Preparation Example 1> Preparation of flake Sendust (FS) particles

In one embodiment of the present invention, a fine forging process (microforging process) was used to plate the magnetic grains, and Sendust alloy was used as the raw material magnetic particles used here. This sendust is a Fe-Si-Al alloy composed of 85% Fe, 9% Si, and 6% Al. The average particle size of raw particles (bulk sandust, BS) is about 40 μm, but larger particles can also be used. The bulk particles were finely forged through a microforging process to produce plate-like flake Sendust (FS) particles.

FIG. 2 shows the shapes of carbonyl iron (CI) and bulk Sendust (BS) used as comparative examples, and flake Sendust (FS) particles subjected to a forging process.

As can be seen in FIG. 2, the CI particles have a spherical shape and the BS particles have an irregular circular shape (approximately average size 40 ± 10 μm), while the FS particles have a disc shape. The average aspect ratio of FS disks with a flat surface and thin thickness is about 50. Although these configurations are the same, FS and BS, which have undergone the microforging process through the microforging process, have different densities. The density of BS is 7.0 g/cm ³ and that of FS is 7.2 g/cm ³ . In general, BS particles have air gaps inside the particles, and these air gaps are removed by a fine forging process during the production of FS particles, so that the density of FS becomes higher than that of BS.

<Production Example 2> Composite Sendust (FS@Co _0.4 Fe _0.4 Ni _0.2 ) Manufacture of ferromagnetic particles

In order to increase the magnetic effect of the flake Sendust (FS) prepared in Preparation Example 1, other ferromagnetic particles were synthesized in a size of several micrometers or smaller than 1 micrometer and attached to the surface of the FS. In the present invention, as an embodiment, Co0.4Fe0.4Ni0.2 having high saturation magnetization was synthesized and FS@Co0.4Fe0.4Ni0.2 composite particles attached to the FS prepared in Preparation Example 1 were prepared. More specifically, it can be produced simply by synthesizing Co0.4Fe0.4Ni0.2 in a solution in the presence of FS. First, 3 g of flake Sendust (FS) prepared in Preparation Example 1 was ultrasonically treated and dispersed in an ethanol/H 2 O mixed solvent (400 ml, 3: 1 v: v ratio), and CoCl 6 H 2 O as a precursor under a nitrogen atmosphere (4.953g), FeCl2 4H2O (4.137g), and Ni(NO3)2 6H2O (3.027g) were dissolved at room temperature, then 6M NaOH solution (5.8ml) was injected, the solution temperature was set to 80℃, and hydrazine Monohydrate (30 ml) was injected. After continuing the reaction for 45 minutes under a flowing nitrogen atmosphere, the final product was recovered using a permanent magnet, washed several times with distilled water and ethanol, and vacuum dried at 25° C. for 24 hours. At this time, the weight ratio of FS and Co _0.4 Fe _0.4 Ni _0.2 was added to be 1:1 based on the amount of the precursor of FS and Co _0.4 Fe _0.4 Ni _0.2 .

Figure 3 is a scanning electron microscope image of the surface of the shape anisotropic particle to which other ferromagnetic nanoparticles are attached to the surface according to an embodiment of the present invention (a) pure FS (flake sandust), (b) synthesized Co _0.4 Fe _0.4 Ni _0.2 particles, (c) FS@Co _0.4 Fe _0.4 Ni _0.2 , and (d) enlargement of the surface of (c).

3 is an electron micrograph showing the shape of the nanocomposite particles prepared in this way, and it can be seen that Co _0.4 Fe _0.4 Ni _0.2 particles having a micrometer size or less are attached to the surface of the FS.

<Production Example 3> Production of magnetorheological body

Ferromagnetic particles of plate-shaped sendust (FS) and composite sendust (FS@Co _0.4 Fe _0.4 Ni _0.2 ) prepared in Preparation Examples 1 and 2 are mixed with 5 to 50% of the fluid medium (silicone oil, mineral oil, lubricant, etc.) By dispersing in a volume ratio, a magnetorheological body was prepared. In the present invention, silicone oil was used to disperse particles in a volume ratio of 10%, but this is only one example and the present invention is not limited thereto.

<Analysis Example 1> Magnetization curve analysis using a vibrating sample magnetometer

The magnetization curves of the particles prepared in Preparation Examples 1 and 2 prepared according to one embodiment of the present invention are 10 kOe (-795.8 kA / m) to 10 kOe (795.8 kA / m) in the magnetic field range of the vibrating sample magnetometer (VSM; VSM-7410, Lake Shore Cryotronics, USA) was used to analyze the magnetization hysteresis of the magnetorheological fluid according to the magnetic field.

Figure 4 shows (a) vibrating sample magnetometer (VSM) data according to an embodiment of the present invention: pure CI (7.8 g/cm ³ ), BS (7.0 g/cm ³ ), FS (7.2g) /cm ³ )particles (1kOe = 10 ³ /(4π) kA/m) (b) Co _0.4 Fe _0.4 Ni _0.2 ,FS,andFS@Co _0.4 Fe _0.4 Ni _0.2 VSM measurement results are shown.

Magnetic hysteresis (hysteresis) curves measured in magnetic fields ranging from -10 kOe (-795 kA/m) to 10 kOe (795 kA/m) are shown in Fig. 4(a). Details of the magnetization hysteresis curve are summarized in Table 1 below.

Psalter CI BS FS Saturation magnetization value [emu/g] 208 120 130 Coercivity [Oe] 21.02 36.04 14.14 Residual Magnetization [emu/g] 3.48 1.20 5.23

The maximum magnetization (saturation magnetization, Ms) values were 208 emu/g for CI, 120 emu/g for BS, and 130 emu/g for FS, respectively. The Ms of CI is greater than that of FS or BS because of its higher Fe content (CI consists of more than 99.5% Fe, whereas Sendust particles contain 85% Fe). For the same reasons as mentioned above, the difference in Ms values for BS and FS is due to the existence of air gaps in the BS or non-magnetic parts. The magnetization curves of CI and BS show an S-shaped hysteresis, which is a typical behavior of magnetic particles with multiple domains.

However, the hysteresis curve (hysteresis) of the FS curve is clearly different from that of CI and BS. Its behavior is similar to that of a perfect (defect free) ferromagnetic material with a large crystal anisotropy. Almost no difference is observed between the Ms values of FS and BS under strong magnetic fields. However, the magnetic moment of the FS particle increases very rapidly under a weak magnetic field, which is more clearly shown in the inset of Fig. 4(a).

The larger induced magnetic moment for FS than for BS or CI is related to the demagnetizing factor. The demagnetization factor is a geometrically dependent constant of magnetization for determining the internal and external values of the magnetic field induced in a magnetic particle by an applied magnetic field. The effective magnetic field acting inside the material for magnetization is equal to the demagnetization field proportional to the demagnetization coefficient. During the fine forging process, a large shape anisotropy in the facile axial direction is created in the sheet plane parallel to the forging direction (Fig. 2(c)). Since the disk-shaped FS particle has a smaller axial demagnetization factor than the spherical particle, the FS is more easily magnetized along the long direction (axial direction parallel to the plane). The plot of the magnetic moment versus the applied magnetic field is linear in the range of -400 to 400 Oe (inset of Fig. 4(a)).

Since the demagnetization field is proportional to the magnetization, the reciprocal of the slope of the linear part at which the field approaches zero gives the experimental demagnetization coefficient. For spherical particles, the demagnetization factor is 1/3 in all directions. If the slope of the experimental demagnetization coefficient of CI is 1/3, the demagnetization coefficients of FS and BS are calculated as 0.05 and 0.24, respectively. This means that 95% of the applied field is used to magnetize the FS particle in the facile axial direction, whereas only 76% of the applied field is used to magnetize the BS particle in the facile axial direction. Therefore, the magnetic moment of FS rises rapidly at low magnetic fields. In general, the smaller the demagnetization coefficient, the larger the coercive field. However, the FS particle has the smallest coercive force despite having the smallest extinction coefficient among the three particles. When an external magnetic field is applied, the magnetic domains align along the direction of the magnetic field. In the absence of an external magnetic field, the magnetic domains of multilayer magnetic materials are oriented rather randomly, but more in the outer layers than in the inner layers. Because the FS particle has a thin disc shape, the magnetic domains of the FS are more uniformly oriented than the BS particle, resulting in a decrease in coercive force.

The magnetic properties of Co _0.4 Fe _0.4 Ni _0.2 , FS and FS@Co _0.4 Fe _0.4 Ni _0.2 are shown in Fig. 4(b). The saturation magnetization of Co _0.4 Fe _0.4 Ni _0.2 , FS and FS@Co _0.4 Fe _0.4 Ni _0.2 particles were 165, 130 and 145 emu/g, respectively. Since the magnetization value of Co _0.4 Fe _0.4 Ni _0.2 is greater than that of FS, attaching Co _0.4 Fe _0.4 Ni _0.2 nanoparticles to FS produces a higher magnetization value than FS. The weight ratio of FS and Co _0.4 Fe _0.4 Ni _0.2 in the nanocomposite particles was set to 1:1. Therefore, the magnetic saturation calculated as a weight ratio is 147.5 emu/g, which is close to the measured value. The curve for Co _0.4 Fe _0.4 Ni _0.2 in the VSM profile in Fig. 4(b) shows a typical hysteresis with a relatively large remanent magnetization (17.0 emu/g) and a slow magnetization increase with increasing magnetic field strength. In contrast, the curve of FS shows a small remanent magnetization (4.6 emu/g) and a sharp increase in magnetization (magnetic sensitivity) with increasing magnetic field strength in the low field strength range (Fig. 4(b) inset). FS has a very small coercive field due to the uniformly oriented magnetic domains in the thin layer. FS@Co _0.4 Fe _0.4 Ni _0.2 represents the mixed magnetic properties of FS and Co _0.4 Fe _0.4 Ni _0.2 . The remanent magnetization (5.2 emu/g) and coercive field values of Co _0.4 Fe _0.4 Ni _0.2 are more similar to those of FS than the average between FS and Co0.4Fe0.4Ni0.2 values despite pre-weighting. In FS@Co _0.4 Fe _0.4 Ni _0.2 , the weight ratio of FS to Co _0.4 Fe _0.4 Ni _0.2 is 1:1. In contrast, the magnetization tendency of FS@Co _0.4 Fe _0.4 Ni _0.2 increased with field strength and was more similar to that of Co _0.4 Fe _0.4 Ni _0.2 than FS. Therefore, the magnetic properties near zero magnetic strength are dominated by the geometric factor of FS, while the properties at higher magnetic fields are determined by Co _0.4 Fe _0.4 Ni _0.2 , which has a higher saturation magnetization value than FS.

<Analysis Example 2> Shear stress analysis of magnetorheological fluid according to magnetic field

Shear stress was measured for the particles prepared in Preparation Examples 1 and 2 prepared according to one embodiment of the present invention using a magnetorheological property measuring instrument (rotational rheometer equipped with a magnetic field generator (Physica MC 300, Stuttgart, Germany)) did

Figure 5 shows the magnetorheological characteristics of a magnetorheological body according to an embodiment of the present invention (a) shear stress and (b) shear viscosity of CI, BS, and FS suspensions for 10 vol% MR fluid at different magnetic field strengths ( c) shear stress as a function of shear rate and (d) viscosity curves for suspensions of FS and FS@Co _0.4 Fe _0.4 Ni _0.2 .

The corresponding flow curves for the CI, BS and FS suspensions are shown in Fig. 5(a) for two different magnetic field intensities of 86 and 343 kA/m. A steady shear curve can be described by dividing the shear rate into two regions. In the case of a general MR suspension, the shear stress of the suspension maintains a constant value at a low shear rate. This is because the mesostructure, which is a fibrous aggregate, is strong enough to withstand the shear force. At high shear rates, the shear stress decreases slightly and then increases due to the destruction and reconstruction of the mesostructure. However, this phenomenon is not clearly observed in the current system. This shows that the particle agglomerates in these suspensions are strong enough to withstand the hydrodynamic stress. At low magnetic field strength (86 kA/m), the FS suspension exhibits the greatest shear stress. It shows a value 10 times greater than that of BS suspension and 5 times greater than that of CI suspension. This is the same as the storage modulus behavior. The low demagnetization coefficient of FS particles enables high shear stress due to their high magnetization. On the other hand, the CI suspension shows the highest shear stress at high magnetic fields, followed by the FS and BS suspensions in the same order as the previous magnetic saturation magnetic force (Ms) values (Fig. 5(a)). The apparent shear viscosity of the suspension (η = τ/(γ) ?, where τ is the shear stress and (γ) ? is the shear rate) is shown in Fig. 5(b) as a function of the shear rate in different magnets. field. Regardless of the magnetic field strength, all suspensions show shear thinning behavior as a result of shear-induced mesostructure destruction. The order of magnitude of shear viscosity was the same as that of shear stress.

Figure 5(c) shows the shear stress and viscosity curves for FS and FS@Co _0.4 Fe _0.4 Ni _0.2 as a function of shear rate (0.1-500 s-1) for two different magnetic fields of 86 and 343 kA/m. show In general, the shear stress curve of MR fluid can be divided into two regions according to the shear rate. At low shear rates, the chain-like structures can withstand the hydrodynamic stress, and the shear stress shows little change in response to changes in shear rate. In contrast, the hydrodynamic stress increases with the shear rate and destroys the mesostructure created by the magnetic field. However, the shear stress curves for the FS or FS@Co _0.4 Fe _0.4 Ni _0.2 suspensions do not clearly show the failure and restoration of chain-like structures that occur at moderate shear rates. This means that the chain-like structures of these two suspensions are robust enough to withstand external shear stress at moderate shear rates. This is a common feature of ferromagnetic particle suspensions and shows the same behavior as the CI and BS suspensions. The trend of shear stress change at different magnetic field intensities is different for FS and FS@Co _0.4 Fe _0.4 Ni _0.2 suspensions. Specifically, the shear stress of the FS suspension at a field strength of 86 kA/m was almost twice that of the FS@Co _0.4 Fe _0.4 Ni _0.2 suspension. When a high-intensity field of 343 kA/m was applied, the FS@Co _0.4 Fe _0.4 Ni _0.2 suspension showed a larger increase in shear stress, and the viscosity increased as the magnetic field strength increased as a result of the shear stress, and FS@Co _0.4 The increase of the Fe _0.4 Ni _0.2 suspension was higher than that of the FS suspension (Fig. 5(d)).

<Analysis Example 3> Analysis of yield stress of magnetorheological fluid according to magnetic field

Figure 6 is the yield stress values of magnetorheological suspensions experimentally measured using a rheometer according to an embodiment of the present invention (a) CI, BS and FS of 10 vol% at magnetic field strengths of 86 kA / m and 343 kA / m Shear viscosity versus shear stress for magnetorheological. (b) Shear viscosity as a function of shear stress for FS and FS@Co _0.4 Fe _0.4 Ni _0.2 MR fluids.

The yield stress τsy is experimentally determined as the point at which the viscosity curve rapidly decreases and can be measured under controlled shear stress (CSS) mode as shown in FIG. 6 . At low shear stress, the viscosity remains high and then suddenly decreases by several orders of magnitude at the static yield point. The measured static yield stress values of FS and FS@Co _0.4 Fe _0.4 Ni _0.2 MR fluids are shown in FIG.

Yield stress (Pa)
Light
transmission (%) H = 86 (kA/m) H =
343 (kA/m) Co _0.4 Fe _0.4 Ni _0.2 (7.85 g/cm ³ ) 1,560 7,052 81 FS (7.20 g/cm ³ ) 2,762 6,078 33 FS@Co _0.4 Fe _0.4 Ni _0.2 (5.75 g/cm ³ ) 1,447 5,412 23 HS-Fe ₃ O ₄ (3.32 g/cm ³ ) 337 588 45

The three yield stress values of each sample were similar. This demonstrates the robustness of the chain-like structure, which does not show an abrupt change in shear stress at low shear rates, as clearly observed in the shear stress curves. Comparing the yield stress of FS and FS@Co _0.4 Fe _0.4 Ni _0.2 shows different trends of yield stress with respect to magnetic field strength for the two samples. For the FS suspension, the measured static yield stress increased ~2.2 times (from 2762 Pa to 6078 Pa) when the magnetic field strength increased from 86 kA/m to 343 kA/m. On the other hand, the static yield stress of the FS@Co _0.4 Fe _0.4 Ni _0.2 suspension increased ~3.7 times (from 1447 Pa to 5412 Pa) with the same field strength change. The yield stress of FS is almost twice that of FS@Co _0.4 Fe _0.4 Ni _0.2 at 86 kA/m, whereas it is only 1.12 times that of FS@Co _0.4 Fe _0.4 Ni _0.2 at 343 kA/m.

These different trends of yield stress increase with magnetic field strength can be explained by the contrasting magnetic properties of the two magnetic materials. As mentioned in the discussion of the VSM results, the magnetization of FS increased much faster than that of FS@Co _0.4 Fe _0.4 Ni _0.2 at low magnetic field strengths and quickly reached saturation values because of the small demagnetization coefficient in the plane-parallel direction. A magnetic field strength of 86 kA/m corresponds to 1.08 kOe in the VSM curve, where the magnetization of FS is substantially greater than that of FS@Co _0.4 Fe _0.4 Ni _0.2 and consequently, compared to FS@Co _0.4 Fe _0.4 Ni _0.2 The yield stress of FS is much larger. On the other hand, the magnetization of FS@Co _0.4 Fe _0.4 Ni _0.2 increases slowly as the magnetic field strength increases. At 343 kA/m, when _both components are almost magnetically saturated, the magnetization _{of FS@Co0.4Fe0.4Ni0.2} is larger than that of FS because of the larger saturation _{magnetization} of _{Co0.4Fe0.4Ni0.2} (Fig _. 3( b)). Therefore, the increase in yield stress response with increasing magnetic field strength should be larger for FS@Co _0.4 Fe _0.4 Ni _0.2 . However, the static yield stress of FS@Co _0.4 Fe _0.4 Ni _0.2 at 343 kA/m is lower than that of FS because there are non-magnetic pores or voids formed between Co _0.4 Fe _0.4 Ni _0.2 particles attached to the particle surface. The voids or pore content was calculated as 24% based on the density of each particle. The yield stress value of FS@Co _0.4 Fe _0.4 Ni _0.2 particle suspension is calculated to be 4966 Pa. Therefore, the additional stress of 446 Pa is due to the synergistic effect of the FS and Co0.4Fe0.4Ni0.2 components (Table 2).

At high magnetic field strengths, the yield stress of the FS@Co _0.4 Fe _0.4 Ni _0.2 suspension is smaller than that of the FS suspension due to the presence of voids or porosity, but substantially larger than that of the Fe3O4 complex-based MR fluid. The rheological properties of the suspension were investigated in a hierarchically structured nano Fe3O4 (HS-Fe3O4) particle suspension prepared by aggregating Fe3O4 nanoparticles. Due to the high saturation magnetization value at the magnetic field strength of 343 kA/m, the measured static yield stress of the FS@Co _0.4 Fe _0.4 Ni _0.2 particles is ~9.2 times greater than that of HS-Fe3O4, even though they are nanocomposite particles. Since the saturation magnetization and density of FS@Co _0.4 Fe _0.4 Ni _0.2 is greater than that of HS-Fe3O4, the magnetization of FS@Co _0.4 Fe _0.4 Ni _0.2 MR fluid is greater than that of HS-Fe3O4 MR fluid, resulting in a high yield stress due to the synergistic effect. It can be confirmed that . The synergistic effect of FS and adsorbed Co _0.4 Fe _0.4 Ni _0.2 nanoparticles uniformly provides excess yield stress.

<Analysis Example 4> Analysis of sedimentation rate of magnetorheological fluid according to magnetic field

In order to see the change in sedimentation rate of the magnetorheological fluid prepared in Preparation Examples 1 and 2 prepared in accordance with one embodiment of the present invention, in which the magnetic particles are dispersed, the magnetorheological in which the prepared magnetic particles are dispersed is tested by Turbiscan. The sedimentation profile recorded for each sample using Turbiscan as a function of time is shown in FIG. 7 .

Figure 7 is an experimental result showing the stability of suspensions according to an embodiment of the present invention (a) transmittance (%) curve as a function of time [h] for CI, bulk sandust and flake sandust for 24 hours. (b) Transmittance (%) curves as a function of time (h) for Co _0.4 Fe _0.4 Ni _0.2 , FS and FS@Co _0.4 Fe _0.4 Ni _0.2 particles for 24 hours.

The light transmittance for all three suspensions (CI, BS, FS) increases rapidly within 2-3 hours. The measured transmittance gradually increases over time and eventually stabilizes. After 24 hours the Turbiscan recorded light transmittances of 72.6%, 69.0% and 29.6% for the CI, BS and FS suspensions, respectively. Although the density of FS is greater than that of BS, the FS suspension is surprisingly much more stable than the density of the BS suspension. Almost 70% of the FS particles did not settle to the bottom after 24 hours. This behavior is due to the disc shape of the FS particle. Plate-shaped particles have a higher coefficient of fluid resistance than spherical particles due to their large surface area, thereby lowering the sedimentation rate and increasing the stability of the suspension accordingly.

For the FS and FS@Co _0.4 Fe _0.4 Ni _0.2 MR suspensions, the same long-term stability was measured by the change in light transmittance using Terviscan (Fig. 7(b) and Table 2). The light transmittance of these suspensions also tends to increase rapidly at the initial stage of the test and then increase very slowly. After the initial stage, no significant change in light transmittance was observed until 24 hours, and the final transmittance values at 24 hours were 73% for Co _0.4 Fe _0.4 Ni _0.2 , FS, FS@Co _0.4 Fe _0.4 Ni _0.2 and 23%, which means that the amount of particles remaining in the medium was 27%, 67% and 77% for Co _0.4 Fe _0.4 Ni _0.2 , FS suspension and FS@Co _0.4 Fe _0.4 Ni _0.2 . When comparing the transmittance value of the FS particle suspension (33%) with that of the FS@Co _0.4 Fe _0.4 Ni _0.2 suspension (81%), the latter shows significantly improved stability despite the relatively high density value of the FS particle (7.2). was Co _0.4 Fe _0.4 Ni _0.2 particles have low stability due to their high density (7.85 g/cm3) (rapid sedimentation of the particles), but show significantly improved stability compared to FS@Co _0.4 Fe _0.4 Ni _0.2 particle suspension and FS suspension. It is exceptional that, in addition to the effect of the shape of the FS particles on precipitation (Fig. 7 (a)), Co0.4Fe0.4Ni0.2 nanoparticles are attached to the surface of the FS particles (Fig. 2 (d)), resulting in the frictional resistance of the surface. because it grows In addition, the air gap between the nanoparticles also helps to improve stability.

The magnetorheological fluid prepared in the present invention is composed of plate-shaped ferromagnetic and paramagnetic particles and oil medium, and when an external magnetic field is applied, the particles are arranged by polarization on the inside and surface of the particles and have a fibril structure ) is formed to improve the viscosity and hinder the flow of the fluid, resulting in yield stress. At this time, the yield stress increases according to the strength of the magnetic field, and when the applied shear stress is greater than the yield stress of the fluid, the fluid flows . In the magnetorheological material of the present invention, magnetic particles are formed in a plate shape, so a demagnetization (non-magnetization) factor acts, so that the magnetization in the plane direction acts much stronger, and the magnetic polarization phenomenon rapidly increases under a low magnetic field, so that other isotropic particles Strong magnetic polarization occurs and strong agglomerates are formed between the particles, which has the characteristics of a magnetorheological body having a high yield stress at a low magnetic field strength. can be reduced Furthermore, by attaching different types of ferromagnetic nanoparticles to the surface of these plate-shaped particles, surface roughness is increased to increase fluid resistance, thereby improving stability, and at the same time, a more efficient and stable magnetorheological by having a synergistic effect due to the increase in magnetism of the nanoparticles. can be manufactured. The response speed of this magnetorheological fluid to the magnetic field is very fast and reversible at the level of milliseconds (10 ^-3 seconds), and stability is remarkably improved. It will be widely used in the development of various equipment such as robotic systems. In addition, since most of these high-performance magnetorheological fluid particles are conductive particles, they can be applied as an electric fluid that reacts under an electric field by mixing with other insulating particles within a range that does not cause an electrical short with a much smaller content.

Claims (11)

a magnetic particle plate-shaped process of forming plate-shaped magnetic particles by using a fine forging process of magnetic particles; and
A method of producing magnetic particles having magnetic anisotropy, comprising a nanoparticle attachment step of attaching the ferromagnetic nanoparticles to the surface of the plate-like magnetic particles through a reduction reaction of the metal salt mixture, which is a precursor of the plate-like magnetic particles and the ferromagnetic nanoparticles, in the presence of a solvent. The method of claim 1,
The magnetic particles are magnetic anisotropy, characterized in that at least one selected from the group consisting of iron, carbonyl iron, iron alloys, iron oxide, iron nitride, carbide iron, low carbon steel, nickel, cobalt, mixtures thereof, and alloys thereof. Method for producing magnetic particles having a. The method of claim 1,
The ferromagnetic nanoparticles are one or more nanometer-sized ferromagnetic particles selected from the group consisting of iron, carbonyl iron, iron alloys, iron oxide, iron nitride, iron carbide, low carbon steel, nickel, cobalt, mixtures thereof, and alloys thereof. Method for producing magnetic particles having magnetic anisotropy, characterized in that the particles. It is prepared according to any one of claims 1 to 3,
Magnetic particles having magnetic anisotropy, characterized in that ferromagnetic nanoparticles are attached to the surface of the plate-shaped magnetic particles. A magnetic particle manufacturing process of manufacturing magnetic particles having magnetic anisotropy by attaching ferromagnetic nanoparticles to the surface of the plate-shaped magnetic particles according to any one of claims 1 to 3; and
Method for producing a magnetorheological fluid comprising the step of dispersing the magnetic particles having magnetic anisotropy in a fluid. The method of claim 5,
The method of producing a magnetorheological fluid, characterized in that the fluid is selected from the group consisting of lubricating oil, mineral oil, silicone oil, castor oil, paraffin oil, vacuum oil, cone oil and hydrocarbon oil. The method of claim 5,
The method for producing a magnetorheological fluid, characterized in that the magnetic particles having magnetic anisotropy are dispersed in 5 to 50% by volume in the fluid. Magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of plate-shaped magnetic particles; and a fluid;
Magnetorheological fluid, characterized in that the magnetic particles having magnetic anisotropy are dispersed in 5 to 50% by volume in the fluid. The method of claim 8,
The plate-shaped magnetic particles and ferromagnetic nanoparticles attached to the surface are selected from the group consisting of iron, carbonyl iron, iron alloys, iron oxide, iron nitride, carbide iron, low carbon steel, nickel, cobalt, mixtures thereof, and alloys thereof one or more species;
Magnetorheological fluid, characterized in that the fluid is selected from the group consisting of lubricating oil, mineral oil, silicone oil, castor oil, paraffin oil, vacuum oil, cone oil and hydrocarbon oil. The magnetorheological fluid according to claim 8 or 9 is used for suppressing or controlling shock or vibration of suspensions, clutches, shock mitigation devices, vibration control devices, earthquake-resistant devices, and polishing devices. Magnetic particles having magnetic anisotropy in which ferromagnetic nanoparticles are attached to the surface of plate-shaped magnetic particles; Including the heat transfer body particles and the fluid,
Electrorheological fluid, characterized in that the magnetic particles having magnetic anisotropy are dispersed in the fluid at 5 to 50% by volume.