What’s a “magnetohydrodynamic seal”?

1 Introduction Since the mid-1980s to the present, many enterprises in China that produce monocrystalline silicon have successively introduced advanced foreign Czochralski crystal growth furnaces. The rotary seal used in these devices is a new type of seal – the magnetohydrodynamic seal. http://www.solar168.cn/Files/product/single_crystal_furnace/pic1 Magnetic fluid seal structure diagram.JPG A semi-fluid dynamic seal that uses ferrofluid to block leakage paths (see the schematic diagram of the magnetic fluid seal). Ferromagnetic ultra-fine particles such as Fe3O4‧CrO2 are suspended in a low-volatility liquid to form a ferrofluid. When ferrofluid is placed in a sealed gap, it can form a strong and resilient liquid film under the influence of magnetic forces, thereby preventing leakage. Magnetic fluid seals are commonly used to seal gases and are suitable for sealing high-vacuum systems. It can be used at high speeds, but it cannot withstand high temperatures and pressures. The excellent sealing performance of magnetic fluid seals is ensured by their unique and advanced sealing principle, which enables long-term stable sealing in devices ranging from ordinary ones to high-precision, cutting-edge instruments. Compared to other sealing methods, magnetic fluid sealing is a type of seal that was invented and put into use relatively late. However, with the development of society and advancements in human science and technology, it differs from certain other sealing methods such as mechanical seals, packing seals, floating ring seals, and labyrinth seals. Its sealing performance can no longer meet the requirements for sealing in today’s society, and it is gradually being phased out of various devices. On the other hand, magnetohydrodynamic sealing has received more in-depth research and application, and it is expanding into many different fields. The production equipment for semiconductor material monocrystalline silicon is the crystal growth furnace, which requires very high levels of sealing to ensure that the entire crystal remains free from air contamination during the growth process. And the most critical of these are the rotating seal of the seed crystal rod at the upper part of the single-crystal furnace and the rotating seal of the crucible rod at the lower part, namely the so-called dynamic seals. Magnetic fluid sealing is required at these two locations. Below is an introduction to its sealing principle and main performance characteristics. 2 Principle of magnetorheological sealing: Magnetorheological sealing relies on the strong paramagnetism of magnetorheological fluid; under the influence of an external magnetic field, this fluid remains in a flowable state as it moves along the magnetic field lines within the sealing gap. Figure 1 is a schematic diagram of the structure of the magnetohydrodynamic 1-body sealing device. The structural diagram of the magnetohydraulic seal device consists of five main components: a magnet, magnetic poles, an outer sleeve, magnetohydraulic fluid, and a rotating shaft. Under normal circumstances, the rotating shaft is the moving component, while the rest of the components are designed to be stationary; a certain gap is maintained between the stationary and moving components, and this gap is filled with magnetic fluid. A magnetic circuit is formed by magnetized fluid, magnetic poles, magnets, and a rotating shaft. The magnetic field always exists within such a continuous magnetic circuit, with virtually no magnetic leakage; a certain magnetic field strength is maintained stably within this circuit over time. The magnetized fluid remains permanently in the gap, thereby serving as a dynamic seal. In this gap, the magnetofluid repels all external non-magnetic materials, which are the media that we need to seal. These non-magnetic substances include gases, liquids, and solids. Solids generally refer to relatively small solid particles that are flowable. In our applications, whether under positive or negative pressure, the sealing is composed of multiple stages; only in a pressure-free environment, such as for dust protection, is a single-stage seal used. Figure 2 shows the operation of the multi-stage seal system. The total pressure difference that the seal can withstand is P1–P6. In the latter stages of this seal system, P4 = P5 = P6, meaning there is no pressure difference; thus, the pressure-bearing capacity is provided by the first three stages, while the last two stages are in a \"standby\" state and serve as backups. It can be seen here that when the pressure difference is large, the number of sealing stages required increases accordingly. The seal grades for the seed crystal rod seal and the crucible rod seal in the single-crystal furnace are set at 4 levels, which are determined by the operating pressure of the equipment and the seal lifespan. The pressure range inside the single-crystal furnace is ±0.1 MPa, and the service life of the seal under continuous use can exceed 10 years, which is several times, or even a dozen times, longer than that of other sealing types; this has led to a correction in the perception that sealing devices are fragile components. There are roughly 7 or 8 manufacturers of magnetorheological fluids in China, and their capabilities vary greatly. Most of these companies only have office buildings and outsource all processing tasks to others; they are essentially small workshops. The largest and best-performing company in this field is Zhuzhou Zhongxin Technology Company, which has 15 mu of factory space and equipment in the Liyu Industrial Park in Zhuzhou. Over the past two years, the company’s products have been found to be used in industries where magnetorheological fluids are particularly utilized overseas, such as the microelectronics industry and the liquid crystal recycling industry. 3. Magnetorheological Fluid Sealing Performance 3.1 Pressure Range: Magnetorheological fluid seals utilize a multi-stage structure to share the pressure load. However, it is not possible to increase the number of stages indefinitely in order to raise the sealing pressure; instead, efforts are made to improve the properties of the magnetorheological fluid or to optimize the structural design and magnetic circuit design in order to achieve a certain level of pressure resistance. Generally, magnetic fluid seals are used in pressure ranges below 0.5 MPa, with negative pressures reaching up to 1.3×10-4 Pa gauge; in special cases, even higher values are possible. When we studied its sealing pressure in the laboratory, it reached 0.87 MPa in a stable state, which was achieved under conditions where the axial dimensions and the number of sealing stages were limited. This means that magnetic fluid seals are suitable for use in conditions ranging from low pressure to high vacuum; both sides of the seal can withstand positive or negative pressures, with no requirement regarding direction. 3.2 Applicable temperature: Magnetic fluid seal devices are generally equipped with water cooling to ensure that the magnetic fluid operates at normal temperatures. The temperature of the crystals in a single-crystal furnace is around 1400°C; under normal cooling conditions, the seal device functions properly and is not affected. When a temperature control device is available, magnetic fluid seals can generally be used in the range of -20°C to 75°C; exceeding this range reduces the service life of the seal. 3.3 Linear velocity: The linear velocity for magnetic fluid seals ranges from 0 to 100 m/s, meaning they can function properly at low speeds as well as achieve satisfactory sealing performance at high speeds. The magnetohydrodynamic sealing device we developed in 1984 has been applied in equipment in the chemical industry, where the rotation speed reaches 8,000 revolutions per minute and it is used under vacuum conditions. Other forms of sealing are very difficult to achieve. This is because magnetic fluid sealing is a non-contact type of seal; at extremely high speeds, frictional heating is minimal, and there is no wear, so power loss is very low. Relative vibration and noise are also reduced. 3.4 Leakage rate The magnetorheological fluid sealing device is a multi-stage seal. The first few stages of seals that are in contact with the medium pressure are responsible for bearing the entire pressure, while the subsequent stages of seals are in a “standby” state and do not participate in the sealing process; there is no pressure difference, so the leakage rate of the medium is practically zero. When using a helium mass spectrometer for leak detection, no leakage is detected, and this is something that other types of seals cannot achieve. During the production process, the single-crystal furnace operates under vacuum or argon protection; magnetic fluid is used to seal the two rotating components, which reduces the air leakage rate of the entire equipment and ensures product quality. 3.5 Maintenance: Magnetic fluid seals require much less maintenance compared to contact seals such as mechanical seals, packing seals, and oil seals, because magnetic fluid seals have a long service life. A unit in Beijing purchased 5 single-crystal furnaces from an American company starting in 1985, and has been using them to produce monocrystalline silicon; under normal circumstances, no maintenance work is required. Only in one of the single-crystal furnaces used in 1987 did the seed rod seal leak in 1991, causing the entire equipment to cease functioning. I am involved in semiconductor technology; in the April 1997 issue, No. 2, we inspected this sealing device. Since there were no holes for adding magnetofluid on its outer surface, we had to disassemble the entire sealing device. There was very little magnetofluid left, and it could not withstand the pressure difference. The other components were in good condition. By adding some vacuum-compatible magnetofluid that we manufactured, the sealing device was able to be put back into use, and it has been functioning well ever since. Over the years, I have also carried out maintenance on the magnetohydrodynamic seals used in single-crystal furnaces in other facilities, mainly by replenishing the magnetohydrodynamic fluid in those seals. Therefore, in normal use, magnetic fluid seals require no maintenance, repair, or replacement of components; the magnetic fluid can be replenished depending on usage. 3.6 Sealing Dimensions As can be seen from the structural diagram of the magnetohydraulic seal, the main components are the magnetic poles and magnets. Thanks to the different designs of the structures of these two components, the dimensions of the entire sealing device can be kept within a very narrow range, both axially and radially, in order to meet the size requirements of the sealing areas in machinery and equipment. Furthermore, when the external dimensions of the sealing area are large, such as in the rotating shafts of large-scale equipment or in the seals on small shafts in instruments, magnetic fluid sealing is a suitable solution. Magnetic fluid sealing also has several advantages; Table 1 provides a comparison of the characteristics of magnetic fluid sealing versus other types of sealing. In summary, when selecting a seal for use in a particular device, it is necessary to consider not only the ability of that seal to prevent the leakage of medium pressure, but also factors such as the amount of leakage, stability, service life, maintenance requirements, and power consumption, before deciding on the appropriate type of seal to use. The use of magnetic fluid sealing in single-crystal furnaces is based on these properties, thereby achieving an excellent sealing effect. This post was last edited by Chensha on 2007-11-24 at 14:30.]

1. Be careful to prevent solvents or corrosives such as acetone, ethanol, acids, bases, water, and oils from entering the magnetohydraulic seal. 2. The water-cooled magnetohydraulic seal should be cooled with water before operation, and then started up. The cooling water must be clean and free of impurities. After the equipment stops operating, water should continue to flow for more than fifteen minutes. 3. Install the magnetorheological seal equipped with a hollow shaft sleeve; pay special attention to the surface quality of the mating surfaces of the connecting shafts. If necessary, vacuum grease can be applied to the mating surface to facilitate smooth insertion. 4. Please use a helium mass spectrometer leak detector for leak detection. 5. Please do not disassemble the magnetohydrodynamic seal casually to avoid unnecessary damage.

It seems that magnetohydrids have quite high requirements regarding the working environment

Can magnetohydrodynamic sealing be used for sealing liquids, especially those containing particles?

It is possible, but attention needs to be paid to the sealing design

It is quite mature for gas and vacuum environments

I’ve learned it.* . . . . . . . . .