We all know that the acquisition of vacuum is inseparable from good sealing. Vacuum sealing technology is an important means to ensure the vacuum degree of the system. Common vacuum sealing technologies include rubber sealing, magnetic fluid sealing, adhesive sealing and metal sealing, etc. Let us take a look at these sealing methods and their characteristics.

In the mid-19th century, the first O-ring appeared. To this day, only more than a hundred years have passed, but the O-ring has a simple structure, convenient loading and unloading, reliable sealing, small dynamic friction resistance, and no need for periodic adjustment, so it has been fully developed and widely used in the sealing of various vacuum systems.

There are two types of rubbers that are more commonly used in vacuum systems: vulcanized rubber made of natural latex, synthetic rubber (including butyl, chloro, nitrile rubber), and silicone rubber, fluororubber, etc.

In addition to having a correct sealing structure design, the reasonable selection of sealing materials is also the key to solving vacuum rubber sealing. Several major factors affecting vacuum sealing are: rubber heat resistance, compression deformation resistance, air leakage rate, air permeability, gas outflow rate, and sublimation (weight loss).

· Heat resistance. In vacuum systems, it is often necessary to degas the system or components, which is usually done by baking. In this way, rubber seals are required to have a certain degree of heat resistance to ensure the smooth progress of baking and degassing. Generally, butyl or nitrile rubber can be used when the baking temperature is below 120°C and the vacuum degree is 10^-5Pa; if a higher baking temperature is required and it works in an ultra-high vacuum environment, fluororubber must be used.

·Compression deformation resistance. In vacuum systems, a large number of vacuum seals work in a compressed state. In order to ensure the reliability of the seal and maintain a certain sealing life, the vacuum sealing rubber should have a smaller compression deformation value (preferably less than 35%), and at the same time, it is required to have a relatively slow degree of compression stress relaxation (that is, a larger compression stress relaxation coefficient), so as to ensure that the vacuum seal has a longer working life.

·Leakage rate. According to experience and calculation, in a vacuum system, when the vacuum pump's exhaust rate is 8000L/s, to maintain a vacuum degree of 5×10^-7Pa, the rubber leakage rate shall not be greater than 5.25×10^-3Pa·cm^3/s. Table 1 below shows the air leakage rate of various rubbers.

· Air permeability. Different rubbers have different air permeabilities to air at different temperatures, which is determined by their internal structures. Nitrile rubber has low air permeability due to the presence of methyl groups; and because nitrile rubber has polar groups of nitrile groups, it has low permeability to non-polar gases. Therefore, the higher the acrylonitrile content of nitrile rubber, the lower its air permeability. It is worth mentioning that temperature has a great influence on the air permeability of rubber. The higher the temperature, the greater the air permeability. In addition, the air permeability of different gases in different rubbers is also different. In the same gas, the order of air permeability is: natural rubber>styrene-butadiene rubber>nitrile rubber>chloroprene rubber>butyl rubber. Table 2 below shows the air permeability of various rubbers to different gases.

· Outgassing rate. The definition of rubber outgassing rate is: the amount of air outflow per unit area of ​​rubber per unit time at a certain temperature. In vacuum sealing, it is generally required to be 10^-4~10^-5Pa·L/s. According to experimental data, various rubbers can be arranged as follows according to the size of the outgassing rate: epichlorohydrin rubber > vinyl silicone rubber > natural rubber > nitrile rubber > chloroprene rubber > fluororubber.

· Rubber sublimation (weight loss). The weight loss of rubber under a certain vacuum degree and temperature is called sublimation. In vacuum sealing, the sublimation value of the sealing material is required to be small. In order to make the rubber seal have a relatively stable relationship in the corresponding vacuum system to ensure the maintenance of the established vacuum degree, the sublimation value is generally required to be less than 10%. According to the size of the vacuum sublimation value, various rubbers can be arranged as follows: natural rubber > nitrile rubber > chloroprene rubber > epichlorohydrin rubber > vinyl silicone rubber > fluororubber. Table 3 shows the weight loss of various rubbers in vacuum.

In high vacuum systems, the main factors affecting the ultimate pressure of the vacuum system by rubber sealing components are the gas leakage rate and outgassing rate of the material.

Magnetic fluid seal

Magnetic fluid was introduced in the 1960s. Magnetic fluid seal is a relatively mature technology and plays an increasingly important role in vacuum sealing.

In principle, magnetic liquid (sometimes called magnetic fluid or ferromagnetic liquid) is a kind of colloidal liquid mixed with solid and liquid, which is made of magnetic nanoparticles, which are specially treated and evenly dispersed in liquid and mixed with it. It has both the fluidity of liquid and magnetism. Magnetic liquid sealing technology is realized by using the response characteristics of magnetic liquid to magnetic field. When we inject well-made magnetic liquid into the gap of magnetic circuit composed of high-performance permanent magnet, pole shoe with good magnetic conductivity and shaft, under the action of magnetic field, magnetic liquid will form several liquid O-rings in the gap, and the number is equal to the number of designed convex teeth. When magnetic liquid is subjected to pressure difference, it will move slightly in non-uniform magnetic field, generate magnetic force against pressure difference, so as to achieve new balance and play a sealing role.

Since there is only contact sealing between magnetic conductor and liquid inside the seal, magnetic fluid seal has the following advantages:

· Tight sealing. Ester-based magnetic liquid can perform tight and stable dynamic and static sealing on medium (atmosphere or inert gas).

· Unmeasurable leakage rate. Basically, it is difficult to detect leakage, so people usually call magnetic fluid seals "zero leakage".

·High reliability. When magnetic fluid seals are under positive pressure and produce instantaneous overpressure breakdown, once the pressure is reduced to the level that the seal can withstand, the sealing effect can still be maintained, and the reliability of use is quite high.

·Basically pollution-free. Since the seal itself does not have mechanical wear and the saturated vapor pressure of the magnetic fluid is extremely low, even if it is used in a high vacuum state, it will not cause pollution.

·Good high-speed performance. When the magnetic fluid is rotating, the internal friction is extremely small and the power loss is small, so it has the potential for high-speed movement.

·Low friction, low wear, and low heat. In seals equipped with bearings, in addition to the small mechanical wear of the bearings during rotation, the internal magnetic core components and the rotating shaft are not in direct contact, so the friction, wear, and heat are low, and the required operating power is small.

·Good repairability. During use, if the magnetic fluid seal fails due to some reasons, as long as the internal components function normally, the magnetic fluid seal can generally be repaired on site in a short time.

·Non-directional seal. If the pressure direction needs to be changed, for magnetic fluid seals, it can be done without adding any components.
Due to the many advantages of magnetic fluid seals, their application areas have involved domestic and imported vacuum equipment such as crystal growth equipment, diffusion furnaces, vacuum brazing furnaces, vacuum heat treatment furnaces and coating machines.
Adhesive seals
In 1985, the Institute of Chemistry of the Chinese Academy of Sciences and the Institute of Electronics of the Chinese Academy of Sciences developed a silicone high vacuum microporous sealant and transferred it to the Nanchang Electrostatic Copying Factory for trial production. This sealant is a colorless, transparent, low-viscosity liquid with strong wettability to metals, ceramics, glass and plastics. It is used to seal micro leaks in high vacuum devices and high vacuum systems with good results. The operating temperature range is 350℃~-196℃.

From the application scenario, there are several types:

① Bonding and fixing various parts in the vacuum cavity;

② Vacuum sealing or plugging. This vacuum glue has a certain strength after curing. It can be used for vacuum sealing at threads, connection and vacuum sealing between glass, ceramics and metals, sealing between electrodes and flanges of feedthroughs/aviation plugs, connection and vacuum sealing of various vacuum tubes such as X-ray tubes and laser tubes, and connection and sealing of vacuum tubes and flanges; In addition, it can quickly seal leaks in various vacuum systems or components. For general vacuum leaks, it can be directly applied to plug leaks. After the glue hardens, it can be directly leak-checked or used normally;

③ Vacuum glue is cured with a curing agent, and there is no irritating odor and volatile substances such as formaldehyde, so it can also be used for non-vacuum applications with high odor requirements.

It can be said that for general vacuum leaks, the adhesive sealing method is very convenient for sealing and plugging leaks, even in ultra-high vacuum, and it can also play a role in connecting some components.

Metal seals

Metal seals have a long history of development and appeared earlier than rubber seals. Relatively speaking, rubber has a larger gas outflow rate and permeability, and cannot be baked at high temperature, and is not resistant to radiation, so its application is subject to certain restrictions. Metal seals make up for the above shortcomings of rubber seals, and are therefore widely used in ultra-high vacuum environments.

In principle, metal materials, like rubber, have certain elasticity and ductility. When the metal sealing ring is elastically deformed by external pressure, the sealing ring tends to restore its original shape under the action of elastic recovery force. This tendency fills the gap on the sealing surface, thereby playing a sealing role.

Here are several metal sealing methods that are often used in ultra-high vacuum.

1. Metal indium wire sealing

The Mohs hardness of metal indium is only 1.2, which is much smaller than the Mohs hardness of metal copper 2.5-3 and aluminum 2-2.9, and the melting point is 156.6℃. Good ductility makes it very conducive to vacuum sealing connection. When sealing, a section of indium wire of suitable length is laid on the flange surface, and the two ends of the indium wire can be overlapped, without the need to be processed into a standard sealing ring in advance. Therefore, it is often used in occasions where the flange size is large and other metal sealing rings are not easy to process.

According to the size of the flange, the diameter of the indium wire can be selected between 1 and 2 mm. However, due to its low melting point, the baking temperature cannot be higher than 150°C. In addition, metal indium has good low-temperature performance, and indium wire sealing is often used in vacuum sealing in low-temperature environments. However, the indium wire is easy to flow after being pressed, so the flange needs to be made into a step or groove type, as shown in Figure 2, to prevent the indium wire from flowing into the vacuum chamber.

2. All-metal quick-release seal

Compared with conventional bolt-tightening flanges that require multiple sets of bolts, the clamp only has two screws to tighten, and can be installed more quickly, so it is called a quick-release clamp. The all-metal quick-release seal mainly consists of a quick-release clamp, a flat flange, and a sealing ring to form a sealing system, as shown in the figure below.

The axial clamping force of the quick-release clamp is small, and pure aluminum is generally used as the material of the sealing ring. In order to reduce the deformation of the clamp, the clamp is made of stainless steel with greater rigidity. The quick-release clamp is composed of a multi-petal clamp, and the number of clamps increases as the flange diameter increases. And as the flange diameter increases, under the same screw tightening force, the clamping force acting on the axial direction becomes smaller. Therefore, for flanges with a nominal diameter less than φ160, the sealing performance of all-metal quick-release seals is better.

All-metal quick-release seals do not require axial screw fixation, so they greatly save axial space, and the number of screws is small and the installation is fast. They are often used in places where the installation space is small and quick installation is required. For example, a high-energy accelerator has a certain dose of radiation just after shutdown. In order to protect the personal safety of the staff, the installation needs to be completed quickly. The all-metal quick-release seal has formed an internal trial standard at the Institute of High Energy Physics, Chinese Academy of Sciences, and has been widely used in the vacuum sealing connection of multiple accelerators.

3. All-metal quick-release and indium wire combined seal

In order to better install the indium wire before installation, the indium wire sealing flange often needs to make a shoulder ring or groove on the flange, as shown in the figure above. In addition, because the indium wire is soft, a larger operating space is required during installation. The all-metal quick-release seal is often prone to small leakage due to insufficient axial clamping force.

The combined seal described in this section inherits the advantages of the soft indium wire and the all-metal quick-release seal that can be installed in a narrow space. The indium wire is installed on the outside of the sealing surface of the all-metal seal ring to solve the problem of gas leakage caused by insufficient clamping force of the all-metal quick-release seal.

This sealing structure is equivalent to extending the path of gas leakage and greatly reducing the flow conductance of the leakage channel. In addition, the indium wire is on the outside of the metal seal ring, which can reduce the risk of the indium wire melting and flowing into the vacuum chamber due to high temperature, thereby improving the reliability of the system.

4. Spring Energy Storage Helicoflex Seal

The Spring Energy Storage Seal (SpringEnergizedSeal) is a pressure-assisted sealing device composed of a metal aluminum, copper, silver, stainless steel or other polymer material jacket and an energy storage spring.

When the spring energy storage seal ring is installed in the sealing groove, the spring is compressed, causing the jacket sealing surface to be close to the sealing groove, thereby forming a seal. The spring provides elastic force to the sealing jacket and compensates for material wear and the offset or eccentricity of the mating parts. The system pressure also assists the sealing jacket to store energy. A constant and lasting preload is formed through the spring elastic force and system pressure, thereby achieving effective sealing.

Spring energy storage seal rings are used for sealing in ultra-high vacuum, nuclear devices, aerospace, petroleum, cryogenics, chemical industry, metallurgy, power machinery, steam containers and other equipment.