Several important concepts in vacuum technology

At one atmospheric pressure, the molecular density of air n = 2.7 × 1019 / cm3. Air is a mixture of gases, the relative molecular weight of which is calculated by the proportion of gas molecules. The molecular weight of nitrogen is 28.016, the volume ratio is 78.1%; the molecular weight of oxygen is 32, the volume ratio is 20.93%; the molecular weight of argon is 39.944, the volume ratio is 0.933%. The average value of relative molecular weight = (28.016 × 78.1 + 32 × 20.93 + 39.944 × 0.933) / (78.1 + 20.93 + 0.933) = 28.96
According to the theory of gas molecular motion, when the pressure is calculated as P = NKT in international standard units (pressure PA, volume m3), the Boltzmann constant k is 1.38 × 10-23 J / K.

The density of gas molecules is n = P / (KT) (1-4) and the pressure is 1pA. When the temperature is 27 ℃, the density of gas molecules is n = 1 / (1.38 × 10-23 × (273 + 27)) = 2.4 × 1020 / m3. Therefore, even if the vacuum reaches a very high vacuum state of 10-9 PA, there are still 1011 gas molecules per cubic meter of space.

1. Average free path of gas molecule
The gas is made up of a large number of molecules. At 0 ℃ and atmospheric pressure, there are 1 mole (6.02 × 1023) molecules in 22.4l space. At room temperature, these molecules move at a speed of 500-1500m / s, collide with other molecules, change the direction and speed of motion, and then collide with other molecules. The flight distance between two collisions is called the mean free path. The relationship among mean free path λ [M], pressure P [PA], temperature T [k] and molecular diameter d [M] is λ = 3.11 × 10-24 T / PD2 (1-5). Therefore, the mean free path of gas molecules is inversely proportional to pressure, temperature and the second power of molecular diameter. The actual data of air, pressure and molecular mean free path at 25 ℃ are 7mm for 1pA, 7cm for 10-1pa, 70cm for 10-2Pa, 7m for 10-3pa and 70m for 10-4Pa. With these numbers in mind, the sense of vacuum can be more intuitive.

2. Incident frequency of gas molecule
The number of gas molecules colliding with solid surface in unit time and unit area is called incident frequency. The relationship between the incident frequency R and the pressure P [PA], the relative molecular mass of the gas Mr, and the temperature T [k] is expressed as R = 2.6 × 10-24 P / (MRT) 1 / 2 [number / (M2 · s)] (1-6), i.e. the incident frequency is in direct proportion to the pressure, in inverse proportion to the square root of the relative molecular mass of the gas and the square root of the temperature. When the vacuum is 10-4 PA, the incident frequency of oxygen molecule at 25 ℃ is 2.72 × 1018 / (M2 · s). The number of atoms arranged on the surface of a solid is about 1019 / m2. In just a few seconds, all the atoms on the surface will be impacted by oxygen molecules.

If the average free path of the gas molecule is the same or longer than the size of the container, the number of collisions between the gas molecule and the container wall is more than that between the gas molecules. This state is called molecular flow (Figure 1.3 (a)). If the average free path is relatively short, the number of collisions between gas molecules is more than that between gas molecules and container wall, this state is called viscous flow (Fig. 1.3 (b)). When the gas molecules of viscous flow are discharged in the pipeline, the flow velocity of the central axis * is large, and the flow velocity near the pipe wall gradually decreases. For molecular flow or viscous flow, different types of vacuum pumps should be selected when obtaining vacuum, which will be described in detail in the vacuum pump section later.

3. Heat conduction in vacuum
If there is a temperature difference between two positions in space, heat conduction will occur. When the gas molecules are viscous flow, the molecular density of heat transfer and the average free path of molecules change with the change of pressure. Because the average free path and molecular density of molecules have opposite effects on heat conduction, the relationship between the contribution of gas to heat conduction and pressure can be ignored for viscous flow.

If the average free path of gas molecules is longer than the distance between two sides of vacuum, the gas molecules that obtain heat energy will move directly through the vacuum to the other side. Therefore, the heat transferred through the gas molecules is related to the molecular density of the gas. The molecular density of gas is in direct proportion to the pressure, so the heat transferred by gas molecules is in direct proportion to the pressure. However, the molecular density in the molecular flow is very low, and the actual heat conduction can also be ignored.

For vacuum environment, the heat conduction of molecular flow state is mainly radiation. In order to improve the film quality, the substrate is usually heated in a vacuum vessel, which is mainly realized by thermal radiation.