Compositions – Fluent dielectric – Metal- or insoluble component-containing; e.g. – slurry,...
Reexamination Certificate
1998-11-27
2001-08-21
Kopec, Mark (Department: 1751)
Compositions
Fluent dielectric
Metal- or insoluble component-containing; e.g., slurry,...
C252S070000, C252S073000
Reexamination Certificate
active
06277306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-rheological fluid, and more particularly to an electro-rheological fluid having high resistance to dielectric breakdown (hereinafter, “dielectric breakdown strength”), and methods of manufacturing and storing the same.
2. Description of the Related Art
An electro-rheological fluid is a fluid that can significantly and reversibly change its rheological characteristics with electrical control. The phenomenon of dramatic change of the apparent viscosity of a fluid through the application of an electric field has been known as the Winslow effect for a long time. The application of this effect to components for electrically controlling devices or parts, such as clutches, valves, engine mounts, actuators, and robot arms has been discussed.
As a result, many proposals have been made on particles or liquid mediums used as a dispersoid for the purpose of obtaining a fluid having a high electro-rheological effect and excellent reproducibility. Conventionally, when a high voltage of 2.5 to 3.5 kV/mm or more is applied to an electro-rheological fluid (hereinafter occasionally referred to as “ERF”) under a flow, it is found that electrical discharge generates frequently such that the fluid cannot be actually used.
It has been discovered that an oil type medium, such as dimethylsilicone or fluorosilicone, and anhydrous particulates such as carbonaceous particulates which form this ERF have dielectric breakdown strengths of 6 kV/mm or more respectively. Accordingly, even if both the oil type medium and the conductive particles used are materials having high dielectric breakdown strengths, it is found that the dielectric breakdown strength of the obtained ERF does not reach a desired level.
When such an electro-rheological fluid is applied to dampers or clutches, a practical vibration controlling effect or the like cannot be obtained due to electrical discharge, and thereby reliability lowers.
An object of the present invention is to provide an electro-rheological fluid made of an oil type medium and fine particulates by mixing, which is highly reliable and in which dielectric breakdown such as generation of electrical discharge hardly occurs at all even if high voltage is applied to the ERF. Further, a second object of the present invention is to provide a method of manufacturing an electro-rheological fluid by which such an electro-rheological fluid is obtained through a simple process and a method of storing an electro-rheological fluid which can prevent reduction in performance of the fluid at the time of conveyance and storage.
As a result of assiduous studies, the inventors have found that, in a state in which air or air forming gas (nitrogen, oxygen, argon, or the like) is included in an ERF made of an oil type medium and fine particulates by mixing, electrical discharge generates when a high voltage of 3.5 kv/mm or more is applied under a flow, and that the above-described objects are achieved by preventing the generation of electrical discharge. The present invention has been thereby completed.
SUMMARY OF THE INVENTION
An electro-rheological fluid of the present invention is an electro-rheological fluid comprising fine particulates dispersed in an oil type medium having an electric insulation property and has a dielectric breakdown strength of 4 kV/mm or more. As a preferable aspect, foaming does not take place when the electro-rheological fluid is placed under a reduced pressure of 10 Pa, or alternatively, 20% or more by volume of a gas contained in the oil type medium is a gas having a dielectric breakdown strength of 4 kV/mm or more.
Further, a method of manufacturing an electro-rheological fluid of the present invention includes a step of stirring and mixing particles and an oil type medium under a reduced pressure of 10 kPa (about 0.1 atmospheric pressure) or less, preferably 1000 Pa or less, or alternatively, a step of degassing a mixture, which was obtained by stirring and mixing particles and an oil type medium and which is disposed under a reduced pressure of 10 kPa or less, preferably 1000 Pa or less. It is preferable that this degassing step takes place while heating the mixture at 40° C. to 80° C. and/or stirring the mixture with rotational stirring blades or with irradiation of supersonic waves, or the like.
In a method of storing an electro-rheological fluid of the present invention, a container for storing the electro-rheological fluid, in which fine particulates are dispersed in an oil type medium having an electric insulation property, is filled with a gas having a strong electron attracting capability and a high dielectric breakdown strength and is thereafter sealed. It is preferable that the gas having a strong electron attracting capability and a high dielectric breakdown strength is of one or more selected from among SF
6
, CCl
2
F
2
, C
3
F
8
, C
2
F
6
, C
5
F
8
, CF
3
CN, C
2
F
5
CN, Cl
2
, SOF
2
, C
2
ClF
5
, and ClO
3
F (each having a halogen atom, a CN group, or a SO group in its molecule).
DETAILED DESCRIPTION OF THE INVENTION
Fine particulates which are preferably used for an electro-rheological fluid of the present invention can be any particle that is known as a particle for an electro-rheological fluid. These include organic semiconductor particulates, carbonaceous particulates, polyurethane particulates, surface-insulated membrane coated particulates, organic and inorganic complex particulates, ceramic particulates, hydrous particulates, or the like.
A preferable example of the fine particulates which can be used in the present invention is a carbonaceous particle. As the carbonaceous particulates, those having a carbon content of 80 to 97% by weight are preferable, and 85 to 95% by weight are more preferable. Further, a C/H ratio (carbon/hydrogen atom ratio) of the carbonaceous particles is preferably from 1.2 to 5, and is more preferably from 2 to 4.
It has been known for a long time that the electrical resistance of the dispersed phase of an electro-rheological fluid is, in general, in a semiconductor domain (W. M. Winslow: J. Appl. Physics vol. 20, page 1137 (1949)), however, carbonaceous particles having a carbon content of less than 80% by weight and a C/H ratio of less than 1.2 are insulating materials, and thus a fluid having an electro-rheological effect can barely be obtained therefrom. On the other hand, those having a carbon content of more than 97% by weight and a C/H ratio of more than 5 are similar to conductive materials and show an excessively large electric current even when voltage is applied, and thus a fluid having an electro-rheological effect cannot be obtained.
Examples of methods of manufacturing these carbonaceous particles include a method of heat-treating pulverizing, and classifying mesophase that was produced by heat-treating pitch or the like, a method of carbonizing a thermosetting resin by heat-treating, and a method of carbonizing aromatic sulfonic acids or a condensation product of a salt thereof, formed in a fine spherical body, by heat-treating in an inert gas atmosphere such as nitrogen, argon, or the like.
The average particle size of the particles can be measured with a particle size analyzer (e.g., MICROTRAC SPA/MK-II TYPE manufactured by Nikldso Co., Ltd.) as mentioned in the Examples. The average particle size of the particles for an electro-rheological fluid obtained after the carbonizing treatment is preferably about 0.1 to 20 &mgr;m, and more preferably 0.5 to 15 &mgr;m. If the average particle size is less than 0.1 &mgr;m, the initial viscosity of the obtained electro-rheological fluid becomes high. If the average particle size is more than 20 &mgr;m, the dispersion stability of the particles deteriorates. Neither is preferable.
The electro-rheological fluid of the present invention is obtained by dispersing the particles for the electro-rheological fluid obtained as described above in an oil type medium having an electric insulation property. The particles for an electro-rheological fluid, which are disperse
Endo Shigeki
Maruyama Takayuki
Saito Tasuku
Wada Hiroaki
Bridgestone Corporation
Hamlin D G
Kopec Mark
Oliff & Berridg,e PLC
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