Compositions – Magnetic – Flaw detection or magnetic clutch
Reexamination Certificate
2000-06-09
2002-08-27
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Flaw detection or magnetic clutch
C252S062550
Reexamination Certificate
active
06440322
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic fluid and a process for producing the same. In particular, the present invention relates to a magnetic fluid suitable for use as a working fluid for dampers, actuators, shaft seals, vacuum seals, dynamic bearings, and the like, and to a process for producing the same.
BACKGROUND ART
A magnetic fluid obtained by extremely stably dispersing fine magnetic metal oxide particles, such as magnetite, in a liquid phase is such a functional fluid that it is flowable but, by the application of a magnetic field, the particles in the liquid can be quickly and reversibly actuated and the fluid can be quickly and reversibly changed in flowability, viscosity, and the like, and be changed into even gel conditions showing no flowability. Since the viscosity of such magnetic fluids can hence be easily controlled by an external magnetic field, use of these fluids as working fluids for various mechanical devices, including dampers, actuators, shaft seals, vacuum seals, dynamic bearings, and the like, has been investigated.
A known magnetic fluid containing a metal oxide is an oil-based fluid prepared by causing magnetite particles to adsorb oleic acid and dispersing these particles in kerosine (JP-A-53-17118).
Furthermore, a technique for obtaining a water-based magnetic liquid is disclosed (JP-A-54-40069). In this technique, magnetite prepared by a wet process is caused to adsorb oleic acid in an aqueous solution thereof, and the resultant flocculate is converted through filtration to a cake having a water content of about 50%. This cake is transferred to a beaker, and a solid powder of sodium dodecylbenzenesulfonate is added thereto, followed by stirring, and thus the filtration cake disperses to rapidly become a low-viscosity liquid to give a water-based magnetic liquid.
A method of dispersing fine iron particles in a mercury matrix by electrodeposition has been used in investigating the magnetism of fine iron particles. By this method, an iron-containing magnetic fluid based on a liquid metal has been obtained (J. Van Wonterghem, S. Morup, S. W. Charles, and S. Wells,
J. Mag. Mag. Master.,
65: 276 (1987)).
And, when such an iron-containing magnetic fluid is exposed to the air, the value of magnetization thereof rapidly decreases due to that the fine iron particles are susceptible to oxidation in the air. Hence, a method and apparatus for obtaining a magnetic fluid containing fine iron nitride particles, which are chemically more stable than iron and have a high saturation magnetization and a high electrical conductivity, have been disclosed (JP-A-3-187907 and JP-A-5-70784). This method comprises introducing a vapor of iron carbonyl (Fe(CO)
5
) simultaneously with N
2
gas into a heater to decompose the Fe(CO)
5
and yield iron nitride (Fe
3
N or Fe
4
N), and thus a magnetic fluid containing iron nitride is synthesized.
However, no magnetic fluid which fully satisfies high magnetism and unsusceptibility to oxidation has been obtained so far. Problems include the following.
Magnetic fluids which are fluids actuating in response to a magnetic field employ an iron-based oxide, metallic iron, or a nitride as ultrafine particles of a magnetic material so as to keep the dispersed particles in colloidal conditions.
Such oxides are insufficient in magnetism, while the metal and nitrides have a problem concerning stability due to that they oxidize in air in several months. Consequently, the practical use of those magnetic fluids is limited to applications in vacuum or inert gases.
For obtaining a mighty magnetic actuation in an oxide-containing magnetic fluid conventionally used frequently, it is necessary to heighten the intensity of an external magnetic field applied thereto, or to heighten the concentration of particles in the fluid, or to use magnetic particles having a larger diameter. However, to heighten the intensity of the applied magnetic field is undesirable from the standpoint of energy consumption. To heighten the concentration of particles is also disadvantageous in that not only the particles, when present at a concentration which is too high, are apt to microscopically aggregate to show impaired dispersibility, but also an external magnetic field does not effectively act on the particles due to that the particles have a shielding effect on each other.
On the other hand, in the case of using particles having a larger diameter, each magnetic particle is not constituted of only one magnetic domain, thereby posing the following problems. These particles magnetically aggregate, and the aggregate particles are too heavy to be dispersed by the thermal movement thereof. As a result, the particles sediment in the solvent to cause phase separation, leading to a reduced or no magnetic effect.
As explained above, no magnetic fluid for general use which has properties sufficient for practical use has been obtained so far.
Problems which should be especially solved are as follows. With respect to the magnetic fluid containing a metal oxide, a fluid which is unsusceptible to oxidation and contains particles having a relatively small particle diameter (5 nm to 15 nm) has been obtained. However, this magnetic fluid has poor magnetism. For example, when this magnetic fluid is used in a pressure seal such as a vacuum seal, the seal should have a multistage structure, and the seal structure per se is large and complicated.
Another problem is that the magnetic fluid containing a metal and that containing iron nitride are susceptible to oxidation although having high magnetism, and cannot hence be used in air, water, or the like.
Accordingly, an object of the present invention is to eliminate the problems described above and provide an excellent magnetic fluid which mightily and precisely actuates by the action of an external magnetic field, the viscosity of which can be significantly increased and be easily and precisely controlled by regulating the external magnetic field applied thereto, and which is excellent also in the unsusceptibility to oxidation and dispersibility of the particles and has a sufficiently wide viscosity range. Another object of the present invention is to provide a process for producing the magnetic fluid.
DISCLOSURE OF THE INVENTION
The present inventors made intensive studies in order to eliminate the problems described above. As a result, they have found that the objects of the present invention are accomplished by preparing magnetic metal particles coated with an oxidation-preventive film by forming an oxidation-preventive film on the surface of ultrafine particles of a magnetic metal, as magnetic particles, or reducing a raw magnetic metal oxide coated beforehand with a hydroxide film and then dispersing the magnetic metal particles coated with an oxidation-preventive film in a solvent.
That is, the invention provides the following:
(1) A magnetic fluid comprising magnetic metal particles coated with an oxidation-preventive film stably dispersed in a solvent, wherein the dispersion conditions are maintained.
(2) The magnetic fluid according to the above (1), wherein the magnetic metal particles coated with an oxidation-preventive film have an average particle diameter of from 5 to 20 nm.
(3) The magnetic fluid according to the above (1), wherein the magnetic metal particles coated with an oxidation-preventive film have a saturation magnetization of from 70 to 200 emu/g.
(4) The magnetic fluid according to the above (1), wherein the magnetic metal particles coated with an oxidation-preventive film comprise iron or an iron-containing alloy as the metallic component.
(5) The magnetic fluid according to the above (1), wherein the oxidation-preventive film has a thickness of from 0.01 to 2 nm.
(6) The magnetic fluid according to the above (1), wherein the oxidation-preventive film is an oxide film.
(7) The magnetic fluid according to the above (6), wherein the oxide film is a silica film.
(8) A process for producing a magnetic fluid, comprising forming a hydroxide film on the surface of raw oxide particles of magnetic meta
Atarashi Takafumi
Fujita Toyohisa
Kim Young-sam
Nakatsuka Katsuto
Koslow C. Melissa
Nittetsu Mining Co., Ltd.
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