Electroviscous fluid

Abstract: An electroviscous fluid comprising an electrically insulating liquid and solid electrolyte particles dispersed therein.

Description: The present invention relates to an electroviscous fluid. The electroviscous fluid is a fluid showing an electric viscosity effect such that upon application of an electric field, the apparent viscosity changes quickly and reversibly. Heretofore, as one of electroviscous fluids, a fluid is known which is composed of a fluid system comprising a continuous phase of an electrically insulating liquid, a dispersed phase of fine particles containing or having adsorbed ions and a small amount of water. This water is adsorbed by the fine particles to form ions in the particles, and when an electric field is applied, the ions will move in the particles and will be maldistributed so that the particles will be polarized, whereby the electroviscous effect is believed to be generated by the cohesive force of the particles due to the electrostatic attraction. Further, with such an electroviscous fluid, it is known that the electroviscosity effect and the electric power consumption vary depending upon the amount of water (US-A-3367872). However, such water contained in the conventional electroviscous fluid restricts the practical application of the electroviscous fluid. For example, at high temperature or in an environment where a high shearing stress is exerted to generate heat, the water tends to be evaporated, whereby no adequate electroviscous effect will be shown. Further, there has been a problem that due to the presence of the water, the apparatus is likely to be corroded, and the power consumption tends to increase. US-A-4772407 describes the use of lithium hydrazinium sulphate as a solid electrolyte, which is not sufficiently stable at high temperatures It is an object of the present invention to provide an electroviscous fluid improved to overcome the above problems. This object can readily be accomplished by dispersing solid electrolyte particles in an electrically insulating liquid. Thus, the present invention provides an electroviscous fluid comprising an electrically insulating liquid and solid electrolyte particles dispersed therein, as defined in claim 1. In the accompanying drawing, Figure 1 is a graph showing the viscosity-increasing effect of the electroviscous fluid of the present invention to an applied electric field, wherein the abscissa indicates the applied voltage (kV·mm⁻¹), and the ordinate indicates the viscosity (Poise). Now, the present invention will be described in detail with reference to the preferred embodiments. The electroviscous fluid of the present invention is prepared by dispersing solid electrolyte particles in an electrically insulating liquid. A solid electrolyte is a substance having a high ionic conductivity at a level equal to an electrolyte solution or melted salt, in a solid state within a temperature range substantially lower than the melting point and is referred to also as a solid ionics, a super ionic conductor or a high speed ionic conductor. It is available not only in a crystal form but also in an amorphous form or a polymer form. Various solid electrolytes are disclosed in "Solid Ionics" coauthored by Tetsuichi Kudo and Kazuo Fueki, published by Kodansha or in "Ceramic Materials for Electronics" edited by Relva C. Buchanan, Marcel Dekker, Inc, New York, Basel. The present invention comprises β-alumina as inorganic solid electrolyte. The solid electrolyte of the invention has an electric conductivity generated by the mobility of ions at the temperature of said electroviscous fluid in use around 10⁻¹ to 10⁻⁸ S/cm, preferably around 10⁻² to 10⁻⁷ S/cm, depending upon the electroviscous effect required. The component elements of the inorganic solid electrolyte of the invention may be partly replaced with other elements, as far as the electric conductivity of said solid electrolyte is in the above described condition. The electrically insulating material can exist in the particles in the amount that doesn't inhibit the electric conductivity of the particle as a whole. Such a solid electrolyte is used in the form of fine particles. The particle size of the fine particles is not particularly limited so long has the fine particles can be dispersed in a stabilized condition in the dispersion medium which will be described hereinafter. However, it is preferred to employ particles having an average particle size of from 0.05 to 500 µm, more preferably from 0.5 to 50 µm. Such solid electrolyte particles contain mobile ions derived from the structures, and contain no volatile component such as water. Therefore, an electroviscous fluid prepared by using such solid electrolyte particles is capable of providing a thermally stable electroviscous effect. Next, the electrically insulating liquid suitable for use as a dispersion medium is preferably the one which is capable of dispersing the above mentioned particles under a stabilized condition and which has a high electric resistivity, such as silicone oil, transformer oil, engine oil, an ester or a dihydric alcohol. The amount of the solid electrolyte particles to the dispersion medium is usually from 5 to 50% by volume, preferable from 10 to 40% by volume. For the dispersion, a usual mixing and dispersing machine represented by a ball mill or a supersonic disperser, may be employed. The method for measuring the electroviscous effect was such that by using a rotating coaxial double cylinder viscometer, the increase in the shearing stress upon application of a voltage between the inner and outer cylinders, was obtained under the same shearing speed (162 sec⁻¹), and it was converted to the change in the viscosity. The fluidity of the electroviscous fluid can be controlled by the voltage to be applied. Therefore, its development in the mechatronics field of computer control is expected in future. Specific examples for its application may be mentioned. In the automobile industry, it may be applied to such parts as clutches, torque convertors, valves, shock absorbers, brake systems or power steerings. Further, in the field of industrial robots, it is being applied to various actuators. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. EXAMPLE 1 β-alumina (manufactured by Kojundo Kagaku Kenkyujo) was pulverized in a mortar to obtain particles having an average particle size of 11 µm, which were then dried at 250°C for 48 hours to thoroughly remove water. Then, 11.24 g of the particles were added to 13.08 g of silicone oil (Toray Silicone SH 200, 10 cs), and the mixture was dispersed and mixed for 12 hours by a ball mill. With respect to the electroviscous fluid of the present invention thus obtained, by using a rotating coaxial double cylinder viscometer, the shearing stress upon application of a voltage between the inner and outer cylinders was measured at the same shearing speed (162 sec⁻¹) (the distance between the electrodes: 1 mm). The results thereby obtained are shown in Figure 1. This electroviscous fluid had a viscosity (initial viscosity) of 0.2 poise when no electric field was applied at 25°C, and the viscosity increased to 6 poise when an electric field with an intensity of 2 kV·mm⁻¹ was applied. Even after being heated at 120°C for 12 hours, it showed exactly the same viscosity, and no change was observed in the properties. Further, this electroviscous fluid was heated, and the viscosity at 62°C was measured, whereby the initial viscosity was 0.1 poise, and the viscosity increased to 3 poise upon application of an electric field with an intensity of 2 kV·mm⁻¹. COMPARATIVE EXAMPLE 1 Silicagel particles having an average particle size of 0.9 µm were dried at 250°C for 16 hours to thoroughly remove water. Then, to 10.00 g of the particles, 1.24 g of an aqueous sodium hydroxide solution having a concentration of 13.1 mol/ℓ was added, and the mixture was added to 18.68 g of silicone oil and dispersed and mixed for 12 hours by a ball mill. This electroviscous fluid had an initial viscosity of 0.5 poise at 25°C, and the viscosity increased to 16 poise when an electric field with an intensity of 2 kV·mm⁻¹ was applied. However, after it was heated at 120°C for two hours, its viscosity decreased to 7 poise upon application of the same electric field. After being heated at 120°C for 12 hours, it showed no viscosity change even when an electric field was applied. As described in the foregoing, the present invention provides an electroviscous fluid which exhibits a constant electroviscous effect within a wide temperature range as compared with the compositions disclosed in the prior art.

Claims: An electroviscous fluid comprising an electrically insulating liquid and β-alumina particles as solid electrolyte particles dispersed therein, said solid electrolyte particles containing no water and volatile components. The electroviscous fluid according to Claim 1, wherein said solid electrolyte particles show electric conductivity of 10⁻¹ to 10⁻⁸ S/cm under the temperature of said electroviscous fluid in use. The electroviscous fluid according to Claim 1, wherein said solid electrolyte particles have an average particle size of from 0.05 to 500 µm. The electroviscous fluid according to Claim 1, wherein the electrically insulating liquid is silicone oil, transformer oil, engine oil, an ester or a dihydric alcohol. The electroviscous fluid according to Claim 1, wherein the solid electrolyte particles are in an amount of from 5 to 50 % by volume to the electrically insulating liquid.

Inventors: HATTORI EIJI, OGURI YASUO, SHIMA KOJI, HATTORI, EIJI, OGURI, YASUO, SHIMA, KOJI
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1990
Main classification: C10M171
IPC codes: F16D35, C10M169, C10N10, C10M171, C10N40