JP5660098B2 - Metal powder for magnetic fluid - Google Patents

Description

The present invention relates to a metal powder for magnetic fluid.

The magnetic fluid is a functional fluid obtained by dispersing magnetic particles or ferrite particles of a ferromagnetic material whose surface is covered with a surfactant in a liquid dispersion medium. In such a magnetic fluid, even if magnetic particles or ferrite particles are magnetized, the dispersed state is maintained without aggregation of the particles. For this reason, the whole magnetic fluid can behave like a magnetic liquid.
Such a magnetic fluid has a property that its viscosity and fluidity change according to an external magnetic field. For this reason, a damper (buffer) capable of freely changing the damping force using this property has been put into practical use.

  Such a damping force variable damper generally includes a cylinder that stores magnetic fluid, a piston that slides inside the cylinder, and a magnetic field forming unit that applies a magnetic field to the magnetic fluid. By adjusting the strength of the magnetic field applied to the magnetic fluid by the magnetic field forming means, the fluid properties such as the viscosity and fluidity of the magnetic fluid can be changed. When the fluid characteristic of the magnetic fluid changes, the resistance when the piston slides in the cylinder changes. As a result, the damping force of the damper can be changed.

Using such characteristics, the damping force variable damper can be used as a shock absorber for an automobile that can select an optimum damping force according to, for example, a road surface condition.
As a magnetic fluid used for such a damping force variable damper, for example, particles whose surface is coated with a surfactant and mainly containing a mixture of iron and ferrite are dispersed in a vegetable oil derivative such as castor oil. A magnetic fluid is disclosed (for example, see Patent Document 1).

  Here, when the damping force variable damper repeats the operation of absorbing the impact, external stress (for example, shearing force) by the piston or cylinder is continuously applied to the magnetic fluid. However, in the magnetic fluid described in Patent Document 1, since the particles contained in the magnetic fluid are easily oxidized, there is a problem that the mechanical properties of the particles are deteriorated and the particles are broken or lost. When the particles in the magnetic fluid are broken or lost, the fluid characteristics of the magnetic fluid change, and an unintended change occurs in the damping force characteristics of the damping force variable damper.

Further, the generation of iron hydroxide between the particles and the surfactant on the surface of the particles causes the separation of the surfactant and the deterioration of the surfactant. This makes it difficult to disperse the uniform particles due to the aggregation of the particles, affecting the fluid characteristics.
In addition, a material having a relatively low saturation magnetic flux density, such as ferrite, may require a significant amount of time to change the fluid characteristics of the magnetic fluid with respect to changes in the external magnetic field. In this case, it becomes difficult to control the change of the damping force characteristic with high accuracy.

JP-T-2004-511094

An object of the present invention is excellent in long-term stability of the fluid properties, and to provide a magnetic fluid metal powder capable of realizing excellent magnetic fluid responsiveness against the external magnetic field.

The above object is achieved by the present invention described below.
The metal powder for magnetic fluid of the present invention is a metal powder used for magnetic fluid,
Fe is contained in a proportion of 90 to 98.612 mass%, Cr is contained in a proportion of 0.1 to 5 mass%, and at least one of P (phosphorus) and S (sulfur) is 0.01 to 0, respectively. Composed of an Fe-based alloy containing at a ratio of 5 mass%,
The short diameter of the metal powder and S [μm], the average value of the aspect ratio defined by S / L when the major axis was L [[mu] m] is characterized in that 0.4 to 1.
Thereby, the metal powder for magnetic fluid which can implement | achieve the magnetic fluid which was excellent in the long-term stability of the fluid characteristic and excellent in the response with respect to an external magnetic field is obtained.

In the metal powder for magnetic fluid of the present invention, the Fe-based alloy preferably further contains Mn (manganese) in a proportion of 0.1 to 2% by mass .

In the metal powder for magnetic fluid of the present invention, the average particle size of the metal powder is preferably 0.1 to 25 μm.
Thereby, the fluid characteristics can be optimized in the magnetic fluid.
In the metal powder for magnetic fluid of the present invention, the maximum particle size of the metal powder is preferably not less than the average particle size of the metal powder and not more than 50 μm.
Thereby, the magnetic fluid which suppressed the particle size variation of the metal powder for magnetic fluids, and was excellent in fluidity | liquidity is obtained.

In the metal powder for magnetic fluid of the present invention, the surface of the metal powder is preferably covered with a passive film.
Thereby, the metal powder for magnetic fluids has excellent oxidation resistance, and can reliably prevent deterioration of mechanical properties over a long period of time. As a result, it is possible to prevent the metal powder for magnetic fluid from being destroyed or broken.
In the metal powder for magnetic fluid of the present invention, the passive film is preferably composed of chromium oxide.

In the metal powder for magnetic fluid of the present invention, the saturation magnetic flux density of the metal powder is preferably 1.7 T or more and 2.05 T or less .
As a result, a magnetic fluid that is excellent in the responsiveness (immediate response and magnitude of change) of the change in fluid characteristics with respect to the change in the external magnetic field can be obtained.
In the metal powder for magnetic fluid of the present invention, the metal powder is preferably produced by a water atomizing method or a high-speed rotating water atomizing method.
As a result, each particle has a shape close to a sphere, and there are few irregularly shaped particles, and metal powder for magnetic fluid with a uniform particle size can be efficiently produced. The magnetic fluid thus obtained Since the metal powder for use is excellent in shape characteristics, it is difficult to cause breakage and defects.
In the metal powder for magnetic fluid of the present invention, the metal powder preferably has a Vickers hardness of 100 or more.

It is a longitudinal section showing an embodiment of a damper of the present invention. It is the elements on larger scale which expand and show some dampers shown in FIG. It is a figure for demonstrating operation | movement of the damper shown in FIG. It is a figure for demonstrating operation | movement of the damper shown in FIG.

Hereinafter, the magnetic fluid and damper of the present invention will be described in detail with reference to the accompanying drawings.
[Damper]
First, before describing the magnetic fluid of the present invention, the damper of the present invention will be described.
1 is a longitudinal sectional view showing an embodiment of the damper of the present invention, FIG. 2 is an enlarged partial view showing a part of the damper shown in FIG. 1, and FIGS. 3 and 4 are dampers shown in FIG. It is a figure for demonstrating operation | movement of. In the following description, the upper side in FIGS. 1 to 4 is referred to as “upper” and the lower side is referred to as “lower”.

  A damper 1 shown in FIG. 1 includes a cylindrical cylinder 2 whose upper and lower ends are closed, a piston rod 31 that extends from the outside of the cylinder 2 through the ceiling 21 of the cylinder 2 and extends into the cylinder 2, A piston 3 is provided at the lower end of the piston rod 31 and slides up and down in the cylinder 2. A magnetic fluid 10 is accommodated in the cylinder 2.

  Such a damper 1 operates to expand and contract between a member connected to the upper end of the piston rod 31 and a member connected to the lower end of the cylinder 2. For example, when the upper end of the piston rod 31 is connected to the body of an automobile and the lower end of the cylinder 2 is connected to a wheel or an axle, the damper 1 is connected to the damper 1 when the distance between the body and the wheel (axle) is expanded or contracted. Stretching force is applied.

In the damper 1, the piston 3 slides in accordance with the expansion / contraction force applied between the members. During this sliding, the piston 3 has a resistance force in a direction that relaxes the expansion / contraction force from the magnetic fluid 10. Is granted. As a result, the piston 3 functions as a shock absorber that relaxes and attenuates the stretching force applied between the members.
The damper 1 includes a coil 4 that is provided in the piston 3 and applies a magnetic field to the magnetic fluid 10 accommodated in the cylinder 2, and a power supply circuit 5 that applies a voltage to the coil 4. . That is, the coil 4 and the power supply circuit 5 constitute magnetic field forming means for applying a magnetic field to the magnetic fluid 10.

  Here, the magnetic fluid 10 will be described in detail later, but its fluid properties (viscosity, fluidity, etc.) change depending on the presence or absence and strength of the magnetic field. For this reason, the fluid characteristics of the magnetic fluid 10 can be adjusted by appropriately setting the presence or absence and strength of the magnetic field by the magnetic field forming means. By utilizing such characteristics, the damper 1 becomes a damping force variable damper capable of controlling the damping force.

Hereinafter, each part of the damper 1 will be described in detail.
The side surface of the cylinder 2 shown in FIG. 1 has a two-layer structure (double-cylinder type), and is composed of an outer outer cylinder 22 and an inner cylinder 23.
The space inside the inner cylinder 23 is divided into a rod side chamber 2 a above the piston 3 and a piston side chamber 2 b below the piston 3.

Further, a first reservoir chamber 25 is provided below the piston side chamber 2b via a base valve 24 provided so as to partition the space inside the inner cylinder 23.
The base valve 24 is provided with an orifice 241 penetrating the base valve 24, and the piston side chamber 2 b and the first reservoir chamber 25 communicate with each other through the orifice 241.

A space between the outer cylinder 22 and the inner cylinder 23 is a second reservoir chamber 26. The first reservoir chamber 25 and the second reservoir chamber 26 are adjacent to each other via the lower end portion of the inner cylinder 23.
In addition, an orifice 231 that penetrates the first reservoir chamber 25 and the second reservoir chamber 26 of the inner cylinder 23 is provided, and the first reservoir chamber 25 is connected to the inner cylinder 23 via the orifice 231. The second reservoir chamber 26 is in communication.

  Such a cylinder 2 is comprised with the material excellent in the mechanical characteristic and oil resistance, for example, is comprised with various metal materials. In the present embodiment, the cylinder 2 is preferably made of a nonmagnetic material. In this embodiment, since the coil 4 for generating a magnetic field is provided in the piston 3, the magnetic field is generated in the magnetic fluid 10 at a portion other than the periphery of the piston 3 because the cylinder 2 is made of a nonmagnetic material. It is suppressed or prevented from being applied. For this reason, in the whole cylinder 2, the fluid characteristic of the magnetic fluid 10 can be made uniform.

Here, the magnetic fluid 10 is obtained by dispersing magnetic particles whose surfaces are covered with a surfactant in a liquid phase dispersion medium, and the whole behaves like a magnetic liquid. The magnetic fluid 10 will be described in detail later.
The piston rod 31 is composed of a highly rigid rod-like member, and extends through the center portion of the ceiling portion 21 of the cylinder 2 to the inside and outside of the cylinder 2.

The piston 3 is composed of a columnar member, and the outer surface thereof is in sliding contact with the inner wall surface of the inner cylinder 23 of the cylinder 2. As described above, the piston 3 partitions the space inside the inner cylinder 23 into the rod side chamber 2a and the piston side chamber 2b.
Two orifices 32 and 33 are provided so as to penetrate the piston 3. The orifices 32 and 33 communicate the rod side chamber 2a and the piston side chamber 2b.

  A valve body 34 is provided on the upper surface of the piston 3 in the vicinity of the upper end opening of the orifice 32. The valve body 34 closes the upper end opening of the orifice 32 so that the magnetic fluid 10 cannot flow through the orifice 32 (closed state), and opens the upper end opening of the orifice 32, and the magnetic fluid 10 flows through the orifice 32. It operates to take a possible state (open state). The valve body 34 is a one-way valve having a function of allowing the flow of the magnetic fluid 10 from the piston-side chamber 2b toward the rod-side chamber 2a and blocking the reverse flow. 1 and 2 show the valve body 34 in the open state.

  The valve body 34 opens and closes using the relative flow of the magnetic fluid 10 with respect to the piston 3 generated by the sliding of the piston 3 with respect to the cylinder 2 as a driving force. In order to shift the valve body 34 from the closed state to the open state, it is necessary to apply a pressure of a predetermined magnitude or more to the valve body 34 by causing the magnetic fluid 10 to flow faster than a predetermined speed. Therefore, the valve body 34 is configured to be open only when the piston 3 slides at a speed equal to or higher than a predetermined speed. With such a valve body 34, the damping force can be varied in the damper 1 depending on whether the sliding speed of the piston 3 is low or high.

A ring-shaped coil 4 is provided inside the piston 3. A part of the outer surface of the coil 4 faces each of the orifices 32 and 33.
The coil 4 is connected to the power supply circuit 5 as described above. When a voltage is applied to the coil 4, a magnetic field is generated around the coil 4 as shown by magnetic field lines (broken lines) in FIG. 2.

The coil 4 has a ring-shaped magnetic core and a conductive wire wound around the magnetic core. The magnetic core and the conductor are electrically insulated from each other by a resin coating layer formed on the surface of the magnetic core or the surface of the conductor. Further, both ends of the conducting wire are connected to the power supply circuit 5 respectively.
The power supply circuit 5 includes a power supply and a conductive wire that connects the power supply and the coil 4. The coil 4 and the power supply circuit 5 constitute magnetic field forming means.

Although not shown, the power supply has a transformer circuit that adjusts the voltage applied to the coil 4. According to this transformer circuit, the voltage applied to the coil 4 can be changed, and the strength of the magnetic field generated by the coil 4 can be changed.
Examples of the material constituting the magnetic core include pure iron, Fe-Si alloys, Fe-Cr alloys, Fe-Ni alloys, Fe-Co alloys, amorphous metals, various soft magnetic materials such as soft ferrite, etc. Can be used.

In addition, these soft magnetic materials constitute a magnetic core in the form of, for example, a laminated body or a compacted body.
In FIG. 1, the coil 4 is provided inside the piston 3, but the installation location of the coil 4 is not particularly limited. For example, the outer cylinder 22, the inner cylinder 23, the base valve 24, etc. of the cylinder 2 are provided. It may be provided, and may be provided at a plurality of locations.
In FIG. 1, a magnetic field is applied to the two orifices 32 and 33 provided in the piston 3 using one coil 4, but a magnetic field is applied using each individual coil. May be. In this case, the damping force of the damper 1 can be controlled more finely and strictly by controlling the operation of each coil independently.

Next, the operation (operation) of the damper 1 shown in FIG. 1 will be described.
First, the compression process of the damper 1 will be described.
Here, as shown to Fig.3 (a), let the state which the damper 1 extended | stretched be an initial state.
When the distance between the upper member 8 connected to the upper end portion of the damper 1 and the lower member 9 connected to the lower end portion is reduced, as shown in FIG. The piston 3 slides downward in the cylinder 2.

At this time, part of the magnetic fluid 10 in the piston side chamber 2 b is pushed by the piston 3, passes through the orifice 241, and is pushed out to the first reservoir chamber 25. Accordingly, the magnetic fluid 10 filled in the first reservoir chamber 25 passes through the orifices 231 and 231 and is pushed out to the second reservoir chamber 26.
Furthermore, a part of the magnetic fluid 10 in the piston side chamber 2b passes through the orifice 33 and moves to the rod side chamber 2a.

In this way, a part of the compressive force between the upper member 8 and the lower member 9 is converted into a sliding driving force of the piston 3 or a driving force of the flow of the magnetic fluid 10, so that the damper 1 is absorbed. As a result, the compressive force can be relaxed and attenuated.
Further, when the sliding speed of the piston 3 exceeds a predetermined speed, the valve body 34 is changed from the closed state to the open state, and the flow of the magnetic fluid 10 is also formed in the orifice 231. Due to the formation of this flow, when the sliding speed of the piston 3 exceeds the predetermined speed, the degree of attenuation can be changed.

Here, when a part of the magnetic fluid 10 in the piston side chamber 2b passes through the orifices 32 and 33, a voltage is applied to the coil 4 by the power supply circuit 5, and FIG. ) Is generated.
When the magnetic field is applied, in the magnetic fluid 10 in each of the orifices 32 and 33, for example, magnetic particles (metal particles) in the magnetic fluid 10 are arranged along the magnetic field lines.

When the magnetic particles are arranged as shown in FIG. 3C, the magnetic particles obstruct the flow of the magnetic fluid 10 flowing through the orifices 32 and 33, and the viscosity of the magnetic fluid 10 increases. As a result, the damping force of the damper 1 increases.
In this way, the fluid characteristics of the magnetic fluid 10 flowing through the orifices 32 and 33 can be changed using the coil 4 and the power supply circuit 5. And the damping force of the damper 1 can be changed. Further, by applying a magnetic field to the magnetic fluid 10 at each of the orifices 32 and 33 having a diameter smaller than that of the cylinder 2, the viscosity of the magnetic fluid 10 can be adjusted more strictly.

Next, the extension process of the damper 1 will be described.
Here, as shown in FIG. 4D, the state in which the damper 1 is compressed is set as the initial state.
As the distance between the upper member 8 and the lower member 9 increases, the piston 3 also slides upward in the cylinder 2 in the damper 1 as shown in FIG.
At this time, a part of the magnetic fluid 10 in the rod side chamber 2a is pushed by the piston 3, passes through the orifice 33, and is pushed out to the piston side chamber 2b.

As the volume of the piston side chamber 2b increases, part of the magnetic fluid 10 in the first reservoir chamber 25 passes through the orifice 241 and flows into the piston side chamber 2b.
Further, a part of the magnetic fluid 10 in the second reservoir chamber 26 passes through the orifices 231 and 231 and flows into the first reservoir chamber 25.
In this way, a part of the extension force between the upper member 8 and the lower member 9 is converted into the sliding driving force of the piston 3 or the driving force of the flow of the magnetic fluid 10, whereby the damper. 1 is absorbed. As a result, the extension force can be relaxed and attenuated.

Here, when a magnetic field is applied to the magnetic fluid 10 flowing through the orifice 33 in the same manner as in the compression process, the fluid characteristics of the magnetic fluid 10 flowing through the orifice 33 can be changed, and the damping force of the damper 1 is changed. be able to.
By continuously performing the compression process and the expansion process as described above, the damper 1 can alleviate the expansion force and the compression force generated between the upper member 8 and the lower member 9.

[Magnetic fluid]
Next, the magnetic fluid (magnetic fluid of the present invention) 10 that can be used for the damper 1 as described above will be described.
The magnetic fluid of the present invention has magnetic particles whose surfaces are covered with a surfactant, and a liquid phase dispersion medium for dispersing the magnetic particles. Among these, the magnetic particles are composed of an Fe—Cr based metal material.

In such a magnetic fluid, the action of the surfactant prevents aggregation of magnetic particles when there is no magnetic field. For this reason, even if the magnetic particles are magnetized or charged, the state in which the magnetic particles are dispersed in the liquid phase dispersion medium is maintained. For this reason, the magnetic fluid can behave like a magnetic liquid as a whole.
As described above, in the damper 1, the damping force can be adjusted by utilizing the change in the fluid characteristics of the magnetic fluid according to the presence or absence and strength of the applied magnetic field.

Here, in conventional magnetic fluids, iron particles such as carbonyl iron, ferrite particles, and the like are used as magnetic particles.
However, when a magnetic fluid is used in a movable part such as a damper, an external stress (for example, a shearing force) is continuously applied to the magnetic fluid each time the damper repeatedly expands and contracts. However, in such a conventional magnetic fluid, the contained magnetic particles are easily oxidized when they come into contact with moisture or outside air in the liquid phase dispersion medium. As this oxidation progresses, the mechanical properties of the magnetic particles deteriorate, leading to the destruction and loss of the magnetic particles. As a result, the fluid characteristics of the magnetic fluid change unintentionally, and the damping force of the damper becomes unstable or deviates from the original damping force.

Carbonyl iron particles and the like have a relatively large coercive force. For this reason, there also existed a problem that the change of the fluid characteristic of a magnetic fluid was delayed with respect to the change of an external magnetic field. For this reason, the damping force of the damper 1 cannot be controlled with high accuracy.
Further, since ferrite particles and the like have a low saturation magnetic flux density, magnetization with respect to an external magnetic field is weak in a magnetic fluid, and a sufficient magnetizing force cannot be obtained.

Therefore, in the ferrofluid of the present invention, as described above, particles made of a Fe—Cr metal material are used as the magnetic particles.
The Fe—Cr-based metal has a property that Cr is oxidized before Fe is oxidized. And since the oxide of Cr is chemically very stable, this makes it difficult to oxidize Fe. As a result, the magnetic particles are prevented from being oxidized.

A film called a passive film is naturally formed on the surface of the Fe—Cr metal. This passive film is generally composed of a metal oxide such as chromium oxide, and is a film excellent in weather resistance and oxidation resistance. Therefore, the magnetic particles having a passive film formed on the surface have excellent oxidation resistance and can reliably prevent deterioration of mechanical properties over a long period of time. As a result, it is possible to prevent the magnetic particles from being destroyed or lost.
Further, in such magnetic particles, oxidation of Fe, which mainly forms the magnetic characteristics, is prevented. As a result, magnetic particles that reliably prevent deterioration of magnetic properties due to oxidation can be obtained.
From the above, the damping force of the damper 1 can be stably controlled over a long period of time by using the magnetic fluid of the present invention including magnetic particles made of Fe—Cr-based metal.

Hereinafter, the components of the magnetic fluid of the present invention will be described in detail.
The Fe—Cr-based metal constituting the magnetic particles is a metal (Fe-based alloy) material containing Fe as a main component and containing Cr.
Specifically, the Cr content in the Fe—Cr-based metal is preferably about 0.1 to 5% by mass, and more preferably about 0.1 to 3% by mass. By making the Cr content within the above range, the effect of improving the oxidation resistance by Cr is sufficiently exerted in the magnetic particles, and the Fe content is relatively low, so that the magnetic properties of the magnetic particles are reduced. It is possible to prevent the characteristics (saturation magnetic flux density and the like) from being significantly lowered.

On the other hand, the Fe content in the Fe—Cr-based metal is preferably about 90 to 99.9% by mass, and more preferably about 95 to 99.9% by mass. By setting the content of Fe within the above range, the magnetic particles are highly compatible with excellent magnetic properties (such as saturation magnetic flux density) due to Fe and oxidation resistance due to Cr.
Further, the Fe—Cr-based metal constituting the magnetic particles may further contain at least one auxiliary agent of P (phosphorus), S (sulfur), and Mn (manganese) as a constituent element. . Such an auxiliary agent can increase the hardness of the Fe—Cr-based metal. For this reason, the magnetic particle comprised with the Fe-Cr type | system | group metal containing an adjuvant becomes the thing excellent in abrasion resistance, and generation | occurrence | production of destruction and a defect | deletion can be prevented more reliably.

  Moreover, these adjuvants can lower the melting point of the Fe—Cr-based metal. For this reason, if the temperature of the molten metal (molten metal) of Fe-Cr system metal is the same, the viscosity of the molten metal can be lowered. As a result, when the magnetic particles are manufactured using a molten Fe-Cr metal such as the atomizing method, it is possible to use a molten metal having low viscosity and high fluidity. Uniform magnetic particles can be obtained.

Since such magnetic particles are unlikely to be broken or deficient, a magnetic fluid containing such magnetic particles exhibits stable fluid characteristics over a long period of time.
In addition, a magnetic fluid with few irregular shapes has high fluidity. For this reason, such a magnetic fluid can react rapidly with respect to the magnetic field provided from the outside. And it becomes the thing excellent in the responsiveness of the change of a fluid characteristic.

  Here, the contents of P and S are preferably about 0.01 to 0.5% by mass, and more preferably about 0.05 to 0.3% by mass. Thereby, it is possible to increase the hardness of the Fe—Cr-based metal while preventing the brittleness of the magnetic particles from significantly increasing. In addition, the temperature of the Fe—Cr-based metal melt can be sufficiently lowered without impairing the magnetic properties of the magnetic particles, and magnetic particles with few irregular shapes and uniform particle diameters can be obtained with certainty.

  The Mn content is preferably about 0.1 to 2% by mass, more preferably about 0.3 to 1.5% by mass. Thereby, the hardness of a magnetic particle can be raised. Further, when S is relatively contained as an auxiliary agent, the high temperature brittleness of the magnetic particles may increase, but by containing Mn in a proportion within the above range, MnS (manganese sulfide) Is generated, and this high temperature brittleness can be suppressed. Therefore, it is possible to obtain a magnetic fluid that does not easily cause breakage or loss of magnetic particles and exhibits particularly stable fluid characteristics over a long period of time.

Such an Fe—Cr-based metal may contain impurities that are inevitably included in the production. In that case, the total content of these impurities is preferably adjusted to 0.5% by mass or less, more preferably 0.3% by mass or less.
The average particle size of such magnetic particles is preferably about 0.1 to 25 μm, and more preferably about 1 to 15 μm. Thereby, the fluid characteristics can be optimized in the magnetic fluid. That is, when the average particle size of the magnetic particles is below the lower limit, when the viscosity of the magnetic fluid becomes too small or the viscosity of the magnetic fluid changes according to the external magnetic field, the amount of change cannot be secured sufficiently. There is a fear. On the other hand, when the average particle size of the magnetic particles exceeds the upper limit, the viscosity of the magnetic fluid may be excessively increased, or the destruction / deletion may be remarkably easily caused.

The particle size distribution of the magnetic particles is preferably as narrow as possible. Specifically, when the average particle size of the magnetic particles is within the above range, the maximum particle size is preferably 50 μm or less, and more preferably 45 μm or less. By controlling the maximum particle size of the magnetic particles within the above range, a magnetic fluid having excellent fluidity can be obtained by suppressing the particle size variation of the magnetic particles.
In addition, said maximum particle size means a particle size with a cumulative weight of 99.9%.

Further, when the short diameter of the magnetic particles is S [μm] and the long diameter is L [μm], the average value of the aspect ratio of the magnetic particles defined by S / L is about 0.4 to 1. Is preferable, and about 0.7-1 is more preferable. The magnetic particles having such an aspect ratio have a shape that is relatively close to a sphere, so that they are less likely to be destroyed or damaged by the shape action. For this reason, the magnetic particle excellent in durability is obtained. In addition, as mentioned above, the shape of a magnetic particle can be approximated to a spherical shape by adding various adjuvants to a Fe-Cr type metal. That is, the aspect ratio can be close to 1.
Such magnetic particles preferably exhibit soft magnetism and have a small coercive force as described above. Specifically, the coercive force of the magnetic particles is preferably 20 Oe (1592 A / m) or less, and more preferably 15 Oe (1194 A / m) or less. The magnetic particles having a coercive force within the above range can reliably prevent aggregation when there is no magnetic field.

Further, the saturation magnetic flux density of the magnetic particles may be as large as possible, but is preferably 1.7 T or more, and more preferably 1.9 T or more. When the saturation magnetic flux density of the magnetic particles is within the above range, a magnetic fluid excellent in the response characteristics (immediate response and the magnitude of the change amount) of the fluid characteristics with respect to the change in the external magnetic field can be obtained.
The hardness of the magnetic particles is preferably as large as possible, but preferably the Vickers hardness Hv is 100 or more, more preferably 150 or more. The magnetic particles having such hardness are particularly surely prevented from being broken or broken.

Such magnetic particles may be produced by any method, but for example, those produced by a method such as an atomizing method or a pulverizing method can be used.
Among these, the magnetic particles are preferably produced by an atomizing method.
The atomization method is a method in which a melt (molten metal) is pulverized by colliding with a cooling medium (liquid or gas). When the molten metal is sprayed or collides with a cooling medium, the molten metal becomes fine droplets, and when the droplets come into contact with the cooling medium, the molten metal is rapidly cooled and solidified. At this time, since the droplet is cooled while naturally falling, the shape is made spherical by its own surface tension. Thereby, since it has a shape close to a sphere and the number of irregularly shaped particles is reduced, it is possible to efficiently produce magnetic particles (magnetic powder) having a uniform particle size. As a result, the aspect ratio of the magnetic particles obtained can be made closer to 1.

Examples of the atomizing method include a water atomizing method, a high-speed rotating water atomizing method, a gas atomizing method, a vacuum dissolution gas atomizing method, a gas-water atomizing method, and an ultrasonic atomizing method.
Among these, as the atomizing method, it is preferable to use a water atomizing method or a high-speed rotating water stream atomizing method. According to these atomizing methods, a medium having a large specific gravity (for example, water) is used as the cooling medium, so that the molten metal can be divided more finely. Thereby, fine magnetic particles having a small average particle diameter can be easily produced.

The magnetic particles are preferably subjected to annealing treatment.
The heating conditions in this annealing treatment are preferably a temperature of 600 to 1000 ° C. × 0.5 to 10 hours, and more preferably a temperature of 700 to 900 ° C. × 0.5 to 2 hours. By performing the annealing treatment under such heating conditions, the magnetic particles in the magnetic powder are annealed, and the residual stress generated during the production of the powder can be relaxed. Thereby, the crack of a magnetic particle accompanying a residual stress, etc. can be prevented reliably. That is, since the stress that is allowed until the magnetic particles deteriorate due to the annealing treatment, the durability of the magnetic particles can be increased.

Moreover, the variation in durability between the magnetic particles can be suppressed by reducing the residual stress.
The surface of such magnetic particles is covered with a surfactant.
The surfactant is not particularly limited. For example, various anionic surfactants such as oleate, carboxylate, sulfonate, sulfate ester salt, phosphate ester salt, amino acid salt, Various cationic (cationic) surfactants such as quaternary ammonium salts, ester types such as glycerin fatty acid esters, ether types such as polyoxyethylene alkyl ethers and polyoxyethylene alkyl phenyl ethers, and fatty acid polyethylene glycols Examples thereof include various nonionic (nonionic) surfactants such as an ester / ether type, and various amphoteric surfactants such as alkylbetaines.

In particular, the surfactant preferably has oleate as a main component. Oleate is strongly bonded to the magnetic particles and has high dispersibility of the magnetic particles in the liquid phase dispersion medium. For this reason, the magnetic fluid excellent in stability is obtained by using an oleate as surfactant.
The surfactant has a hydrophilic part and a hydrophobic part in the molecule. For example, when oil is used as a liquid phase dispersion medium for dispersing magnetic particles, surfactant molecules are arranged along the interface between the magnetic particles and the liquid phase dispersion medium. At this time, the hydrophilic portion of the surfactant molecule is oriented to the magnetic particle side, and the hydrophobic portion is oriented to the liquid dispersion medium side.

By such an action of the surfactant, the magnetic particles can be stably dispersed in the liquid phase dispersion medium without agglomeration when there is no magnetic field.
Examples of the liquid phase dispersion medium include an aqueous dispersion medium such as water, a hydrocarbon oil, a silicone oil, a fluorine oil, an ester oil, an ether oil, and the like.
Of these, those mainly composed of hydrocarbon oil, silicone oil or fluorine oil are preferred. Since these oils are excellent in heat resistance and chemical stability, they are particularly preferably used as a liquid phase dispersion medium for magnetic fluids. That is, a magnetic fluid having excellent durability can be obtained.

The content of magnetic particles in such a magnetic fluid is preferably about 50 to 95% by mass, and more preferably about 60 to 90% by mass. Thereby, while being excellent in fluidity | liquidity, the magnetic fluid which shows sufficient responsiveness with respect to the change of an external magnetic field is obtained.
As mentioned above, although the magnetic fluid and damper of this invention were demonstrated based on suitable embodiment, this invention is not limited to this.
For example, the magnetic fluid of the present invention can be used not only for the damper described above but also for a seal member of a rotating shaft, a speaker, a sensor, and the like.

Next, specific examples of the present invention will be described.
1. Production of damper (Example 1)
[1] First, raw materials having the following composition were melted in a high-frequency induction furnace and pulverized by a water atomization method to obtain metal powder (magnetic particles). Subsequently, classification was performed at 32 μm using a standard sieve having an opening of 32 μm.

<Composition of raw materials>
・ Cr: 0.05 mass%
・ P: 0.009 mass%
・ S: 0.003 mass%
・ Mn: 0.06% by mass
-Fe: remainder-In addition, trace elements (Si, C, etc.) which are unavoidable are included.

In addition, when the obtained metal powder was observed with the scanning electron microscope (SEM), it turned out that each magnetic particle has a shape close | similar to a spherical shape.
Moreover, when the saturation magnetic flux density of the obtained metal powder was measured, the saturation magnetic flux density was 2.05T. The average particle size of the metal powder was 6 μm, the maximum particle size was 32 μm, and the average aspect ratio was 0.5.
Next, the obtained metal powder was annealed at a temperature of 800 ° C. for 1 hour in a hydrogen atmosphere.

[2] Next, the obtained metal powder was coated with oleate ions (surfactant) and dispersed in isoparaffin (liquid phase dispersion medium) to obtain a magnetic fluid.
In addition, the content rate of the metal powder in a magnetic fluid was 80 mass%.
[3] Next, the obtained magnetic fluid was injected into the cylinder of the damper to produce a damper.

(Examples 2 to 13)
A metal powder was obtained and a damper was prepared in the same manner as in Example 1 except that the composition of the raw material was changed to the composition shown in Table 1.
The average particle diameter, maximum particle diameter, average aspect ratio, and saturation magnetic flux density of the obtained metal powder are shown in Table 1, respectively.

(Example 14)
The metal powder is obtained in the same manner as in Example 1 except that the raw material composition is changed to the composition shown in Table 1 and the metal powder production method (pulverization method) is changed from the water atomization method to the gas atomization method. A damper was also produced.
(Example 15)
A metal powder was obtained and a damper was produced in the same manner as in Example 4 except that the annealing treatment was omitted.

(Comparative Examples 1-3)
A metal powder was obtained and a damper was prepared in the same manner as in Example 1 except that the composition of the raw material was changed to the composition shown in Table 1.
The average particle diameter, maximum particle diameter, average aspect ratio, and saturation magnetic flux density of the obtained metal powder are shown in Table 1, respectively.

(Comparative Example 4)
A damper was produced in the same manner as in Example 1 except that carbonyl iron powder produced by the pulverization method was used.
Table 1 shows the average particle diameter, maximum particle diameter, average aspect ratio, and saturation magnetic flux density of the metal powder used.
Classification was omitted.

2. Evaluation of Magnetic Particles and Damper Each of the dampers obtained in each Example and each Comparative Example was subjected to a stretching operation 10,000 times.
When the expansion and contraction operation was first performed 1000 times, the magnetic fluid was taken out from the cylinder, and the metal powder in the magnetic fluid was observed with a scanning electron microscope.

Thereafter, the magnetic fluid taken out was returned to the cylinder, and the remaining 9000 expansion / contraction operations were performed.
After 10,000 stretching operations, the magnetic fluid was again taken out from the cylinder, and the metal powder in the magnetic fluid was observed with a scanning electron microscope.
As described above, the metal powder after 1000 stretching operations and the metal powder after 10,000 stretching operations are evaluated according to the following criteria to determine the durability of the metal powder (magnetic particles). Evaluated.

<Durability evaluation criteria>
◎: The shape of the magnetic particles has not changed substantially before the evaluation. ○: Some of the magnetic particles are broken or defective. △: Many of the magnetic particles are broken or defective. ×: Almost all the magnetic particles. Next, an accelerated test under a high-temperature and high-humidity environment was performed on the metal powder after the expansion / contraction operation under the following conditions according to the conditions specified in JIS C 60068-2-3.

<Conditions for accelerated test at high temperature and high humidity>
・ Temperature: 40 ± 2 ℃
・ Relative humidity: 93 ⁺² _-3 %
Test time: 4 days And whether or not rust has occurred in the metal powder after the test was evaluated according to the following evaluation criteria based on observation with an optical microscope.

<Evaluation criteria for the presence or absence of rust>
None: Almost no rust is observed on the magnetic particles. Yes: Many rust is observed on the magnetic particles. Table 1 shows the results of evaluating the durability of the magnetic particles and the presence or absence of rust.

As a result of the evaluation of the magnetic particles, the magnetic particles used in each example maintained the shape before the evaluation, although some of the magnetic particles were found to be broken or defective. That is, the magnetic particles obtained in each example are excellent in durability and rust prevention power.
Moreover, although the tendency that the saturation magnetic flux density tends to decrease as the Fe content in the magnetic particles decreases, the magnetic particles obtained in each example, except for a part, are all magnetic fluids. The magnetic particles used had a sufficient saturation magnetic flux density.

On the other hand, many of the magnetic particles used in each of the comparative examples were found to have been broken or deficient in many magnetic particles.
In the case of a magnetic particle in which destruction / deletion is observed, the magnetic characteristics of the magnetic particles change from the original characteristics, so that the magnetic characteristics of the entire magnetic fluid also change from the original characteristics. That is, a damper using such a magnetic fluid may deviate from the original buffer performance.
In addition, the magnetic particles used in each example hardly showed rust even after the acceleration test in a high temperature and high humidity environment. On the other hand, in the magnetic particles used in each comparative example, rust was observed on many particles.
Furthermore, it has become clear that the durability of the magnetic particles can be improved by annealing treatment.

  1 …… Damper 2 …… Cylinder 2a …… Rod side chamber 2b …… Piston side chamber 21 …… Ceiling 22 …… Outer cylinder 23 …… Inner cylinder 231 …… Orifice 24 …… Base valve 241 …… Orifice 25 …… No. DESCRIPTION OF SYMBOLS 1 Reservoir chamber 26 ... 2nd reservoir chamber 3 ... Piston 31 ... Piston rod 32, 33 ... Orifice 34 ... Valve body 4 ... Coil 5 ... Power supply circuit 8 ... Upper member 9 ... Lower member 10 ... Magnetic fluid

Claims (9)

A metal powder used in magnetic fluid,
Fe is contained in a proportion of 90 to 98.612 mass%, Cr is contained in a proportion of 0.1 to 5 mass%, and at least one of P (phosphorus) and S (sulfur) is 0.01 to 0, respectively. Composed of an Fe-based alloy containing at a ratio of 5 mass%,
The minor diameter of the metal powder and S [μm], ferrofluid average aspect ratio of the major axis is defined by S / L when the L [[mu] m] is characterized in that 0.4 to 1 Metal powder for use.   The metal powder for magnetic fluid according to claim 1, wherein the Fe-based alloy further contains Mn (manganese) in a proportion of 0.1 to 2 mass%.   The metal powder for magnetic fluid according to claim 1 or 2, wherein the average particle diameter of the metal powder is 0.1 to 25 µm. Maximum particle diameter of the metal powder, the magnetic fluid metal powder according to the average particle claims 1 over is 50μm or less diameter or any one of 3 of the metal powder. The metal powder, the magnetic fluid metal powder according to any one of claims 1 to 4 the surface is covered by a passive film.   The metal powder for magnetic fluid according to claim 5, wherein the passive film is made of chromium oxide. The saturation magnetic flux density of the metal powder, the magnetic fluid metal powder according to any one of claims 1 to 6 or less than 1.7T 2.05T. The metal powder, water atomization method or a magnetic fluid metal powder according to any one of claims 1 to 7 are those prepared by high speed water stream atomizing method. The metal powder for magnetic fluid according to any one of claims 1 to 8, wherein the metal powder has a Vickers hardness of 100 or more.

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