Cryogenics | Cryogenics and Vacuum

According to the state equation of gas PV=nRT, the gas pressure in the cavity is directly related to the temperature of the gas molecules inside it. Therefore, the chamber gas pressure (i.e. vacuum degree) can be changed by adjusting the temperature, and low temperature can be obtained by changing the vacuum.

Increasing vacuum by changing temperature

Table 1 shows the standard composition of the atmosphere. Figure 1 shows the temperature relationship curve of the saturated vapor pressure of various gases. As shown in Figure 1, when the temperature drops to 20K, except for gases that are difficult to condense such as neon, hydrogen, and helium, the saturated vapor pressure of all other components in the air is less than 10-10 torr. Therefore, when the airtight container is cooled to 20K, the vacuum can reach 1.2E-3 torr; when it is cooled to 4.2K, only helium is left, and the vacuum can reach 5E-5torr. The cold surface of the condensation pump can generally be selected at around 14-20K, and the amount of hydrogen, helium and other difficult condensable gases in the system to be pumped can be greatly reduced by flushing with condensable gases or vacuuming with other pumps. A small amount of difficult condensable gases that leak into the system during operation can also be pumped away by other pumps.

[Table 1] Standard atmospheric composition

【Figure 1】Temperature relationship of saturated vapor pressure of various gases

Obtaining low temperature by changing pressure (vacuum degree)

Any physical process accompanied by low temperature absorption of heat can be used for refrigeration. One type is phase change refrigeration, which uses the evaporation process of liquid at low temperature or the melting or sublimation process of solid at low temperature to absorb heat from the cooled object to produce cold. The other type changes the temperature of the liquid pool by changing the pressure of the vapor above the liquid, such as gas adiabatic expansion refrigeration. The high-pressure gas can reach a lower temperature through adiabatic expansion, and the low-pressure gas can be reheated to produce cold. For example: 1) Cryogenic liquids such as liquid nitrogen and liquid helium can achieve low temperatures of 77K and 4.2K. The temperature of the liquid pool can be changed by reducing the pressure of the space above the liquid surface through the exhaust system. The lower the pressure, the lower the temperature. Table 2 and Figure 2 show the corresponding relationship between the saturated vapor pressure and temperature of liquid nitrogen and liquid helium respectively. The lowest pressure that can be achieved by decompression depends not only on the pumping speed of the decompression system, but also on the heat flow reaching the liquid pool. In order to get the lowest possible temperature, it is important to try to reduce heat leakage.

【Table 2】Correspondence between saturated vapor pressure of liquid nitrogen and temperature

【Figure 2】Corresponding relationship between saturated vapor pressure and temperature of liquid helium

2）  Gas isenthalpic expansion refrigeration

Under certain pressure and temperature conditions, when a gas is adiabatically expanded through a throttle valve or an expander, its temperature will decrease, and it may even liquefy. This refrigeration method is often used in gas separation, gas liquefaction, and gas refrigerators. For actual gas, the enthalpy value is a function of temperature and pressure. The enthalpy value of the gas before and after throttling generally changes. This phenomenon is called the Joule-Thomson effect. This is the refrigeration principle of the JT refrigerator.

3) Adiabatic degassing refrigeration of gas

When a certain amount of vaporized gas in a container is adiabatically degassed to the ambient medium through a control valve (or vacuumed), the gas remaining in the container will do pushing work for the released gas, consuming part of its own thermodynamic energy (internal energy), and thus the temperature decreases. This is also one of the principles of a gas refrigerator. The closed-loop sample rack of Femian Company uses GM refrigerator to provide cooling capacity, and GM refrigerator is cooled by absorbing heat through adiabatic expansion of high-pressure helium.

The relationship between vacuum and temperature is also reflected in solid materials

Solids also have saturated vapor pressure. Under certain conditions, molecules with greater kinetic energy on the surface of solids can overcome the binding force of adjacent molecules and escape from the surface of solids directly to become steam molecules (sublimation), thereby generating vapor pressure. The pressure when equilibrium is reached is the saturated vapor pressure. For example, solids such as dry ice and iodine can be directly sublimated from solids to gases. Similar to liquids, solid materials also have different saturated vapor pressures at different temperatures, and they increase with increasing temperature. Compared with low temperature, the vacuum degree of the chamber at high temperature is generally lower. On the one hand, it is caused by the high-temperature desorption of a large number of gas molecules. On the other hand, the saturated vapor pressure of the same material at high temperature becomes higher. Before the solid-gas phase dynamic equilibrium is reached, sublimation dominates, which will also cause the vacuum degree to deteriorate.

   Figure 3 shows the relationship between the saturated vapor pressure and temperature of solid materials. The saturated vapor pressure increases significantly with the increase of temperature. At the same temperature, the saturated vapor pressure of Pb, Zn, and Cd is significantly higher than that of other metals. Therefore, these three metals and their alloys are not suitable for ultra-high vacuum environments.

【Figure 3】 Relationship between saturated vapor pressure and temperature of solid materials