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Magnetic fluid (1) Composition of magnetic fluid Magnetic fluid invented by Papell in the United States in 1995 is a colloidal liquid that stably disperses highly magnetic fine powders (about 100Å) such as magnetite in water, oils, esters, ethers and other liquids. This liquid has the property of not settling or condensing under normal centrifugal force and magnetic field, but can also withstand magnetism and can be attracted by magnets. Magnetic fluid consists of 3 main components: ①Solid ferromagnetic particles (Fe3O4) ； ②Surfactants (stabilizers) that coat particles and prevent them from agglomerating with each other ； ③Carrier liquid (solvent). (2) Characteristics of magnetic fluid Magnetic fluid is a kind of colloidal solution. As a magnetic fluid for sealing, its performance requirements are: Good stability, no aggregation, no precipitation, no decomposition ； High saturation magnetization ； High starting magnetic permeability ； The viscosity and saturated vapor are low, and other requirements such as freezing point, boiling point, thermal conductivity, specific heat and surface tension also have certain requirements. The main factors affecting the stability of magnetic fluid are: The strength of the particles, surfactant and carrier liquid and their reasonable ratio. Stability is the prerequisite for the existence of various characteristics of magnetic fluid. 2. Working principle of magnetic fluid seal. The magnetic circuit composed of annular permanent magnet, pole piece and rotating shaft, under the action of the magnetic field generated by the magnet, concentrates the magnetic fluid placed in the gap between the shaft and the top of the pole piece to form a so-called "O" ring, which blocks the gap channel to achieve the purpose of sealing. This sealing method can be used when the rotating shaft is a magnetic body and when the rotating shaft is a non-magnetic body. The former's magnetic flux is concentrated at the gap and runs through the rotating shaft to form a magnetic circuit, while the latter's magnetic flux does not pass through the rotating shaft, but only forms a magnetic circuit through the magnetic fluid in the sealing gap. 3. Limit conditions: The magnetic fluid seal will be restricted by the following conditions when working:: (1) Evaporation. Magnetic fluid consists of magnetic particles, surfactant and carrier liquid. The evaporation of the carrier liquid is the main factor that determines the limit rotation frequency and service life of the seal. Because the seal works on limited magnetic fluid. For this reason, a carrier liquid with low vapor pressure should be selected to minimize evaporation losses. (2) Temperature rise. Increased temperature causes the magnet to demagnetize and the magnetic fluid to evaporate. Because the temperature increases and the viscosity decreases, the power consumption also decreases, which is a plus. However, as the temperature increases, the magnetic saturation strength decreases, which may also reduce the pressure resistance of the seal. Therefore, the temperature of the magnetic fluid should generally not be higher than 105°C, otherwise cooling measures should be taken. (3) Ultimate vacuum degree. The ultimate vacuum degree of the magnetic fluid seal depends on the volatility of the carrier liquid. The carrier liquid made of di-grease lubricant can meet the requirements of 1.333×10-7 Pa ultra-high vacuum technology. (4)Peripheral speed. Generally, magnetic fluid seals are suitable for operation at high peripheral speeds above 30m/s, and there is no limit mark. However, considering the temperature and heat dissipation, the peripheral speed should be limited to 60~80m/s. At this time, the ultimate pressure resistance capability must also be considered.