Several important concepts in vacuum technology

- Jun 15, 2019-

Several important concepts in vacuum technology

 

At one atmosphere, the molecular density of air n=2.7×1019 /cm3.
Air is a mixture of gases whose relative molecular mass is proportional to the molecules of the gas. The relative molecular weight of nitrogen was 28.016 and the volume ratio was 78.1%. The relative molecular weight of oxygen is 32 and the volume ratio is 20.93%. The relative molecular weight of argon is 39.944 and the volume ratio is 0.933%.
The average relative molecular mass of = (32 x 28.016 x 78.1 + 20.93 + 39.944 x 0.933)/(78.1 + 20.93 + 0.933) = 28.96
According to the kinetic theory of gases, the pressure is calculated as
P = nkT

 

In international standard units (pressure Pa, volume m3), boltzmann constant k is 1.38×10-23 J/ k.
The density of the gas molecules is zero
N = p/(kT) (1-4)
At pressure of 1Pa and temperature of 27℃, the density of the gas molecules is n
N = 1 / (1.38 x 10-23 x + 27) (273) = 2.4 x 1020 / m3
Thus, even at a vacuum of 10-9 Pa, there are 1011 gas molecules per cubic meter of space.

 

1. Average free path of gas molecules

A gas is made up of a large number of molecules. At 0an atmosphere, there is 1 mole (6.02×1023 molecules) in 22.4L of space. These molecules travel at room temperature at 500 to 1,500 m/s and collide with other molecules, change direction and speed, and then collide with other molecules. The distance between the two collisions is called the average free path.
 mean free path Lambda [m], pressure p[Pa], temperature T[K], and molecular diameter D[m]
Lambda = 3.11 x 10 to 24 T/pD2 (1-5)

Thus, the average free path of a gas molecule is inversely proportional to the pressure, directly proportional to the temperature, and inversely proportional to the square of the molecular diameter.
The actual data of pressure and molecular mean free path for air with a temperature of 25℃ show that 1Pa is 7mm, 10-1 Pa is 7cm, 10-2pa is 70cm, 10-3 Pa is 7m, and 10-4 Pa is 70m. Remember these Numbers to make the vacuum feel more intuitive.

 

2. Incident frequency of gas molecules

The number of gas molecules bumping into a solid surface per unit time and area is called the incident frequency. The relation between incident frequency r and pressure p[Pa], relative molecular mass of gas Mr And temperature T[K] is expressed as
R =2.6×10-24 p/(MrT)1/2[m2·s] (1-6)

The number of gas molecules bumping into a solid surface per unit time and area is called the incident frequency. The relation between incident frequency r and pressure p[Pa], relative molecular mass of gas Mr And temperature T[K] is expressed as
R =2.6×10-24 p/(MrT)1/2[m2·s] (1-6)

 

If the average free path of the gas molecules is the same or longer than the size of the container, there are more collisions between the gas molecules and the wall than between the gas molecules. This state is called molecular flow (figure 1.3(a)). If the average free path is short, the number of collisions between gas molecules is higher than that between gas molecules and the wall of the container. This state is called viscous flow (figure 1.3(b)). When the viscous gas molecules are expelled from the pipeline, the velocity of the central axis is the maximum, and the velocity near the wall decreases gradually.


For molecular flow or viscous flow, different types of vacuum pumps are selected to obtain vacuum, which will be described in detail in the vacuum pump section.

 微信图片_20190615155314

FIG. 1.3 schematic diagram of molecular flow and viscous flow

3. Heat conduction in vacuum

Heat conduction occurs when there is a temperature difference between two locations in space. In the case of viscous flow, the molecular density of heat transfer and the average free path of molecules also change when the pressure changes. Because the mean free path and molecular density have opposite effects on heat conduction, the contribution of gas to heat conduction is negligible for viscous flow.
In the case of molecular flow, if the average free path of the gas molecules is longer than the spacing between the two sides of the vacuum, the gas molecules obtaining heat energy will move directly through the vacuum to the other side. So the amount of heat transferred through the molecules of the gas is related to the molecular density of the gas. The molecular density of a gas is proportional to the pressure, so the heat transferred by the molecules is proportional to the pressure. However, the molecular density in the molecular flow is very low, and the actual heat transfer is negligible.
For vacuum environment, the heat conduction of molecular flow state is mainly radiation. In order to improve the quality of the thin film growing in vacuum container, the substrate is usually heated, which is mainly achieved by thermal radiation.

IKS PVD,vacuum coating machine,contact:iks.pvd@foxmail.com

微信图片_20190321134200