The meaning and characteristics of vacuum

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In vacuum science, a vacuum refers to a gas state in a given space where the pressure is below one atmosphere. People usually refer to this state of thin gas as a vacuum condition. Compared to the atmosphere on which human life depends, this specific vacuum state has the following main characteristics: (1) The gas pressure in a vacuum state is lower than one atmosphere. Therefore, various vacuum containers located on the Earth’s surface are subject to atmospheric pressure, and the magnitude of this pressure difference is determined by the pressure difference between the inside and outside of the container. Since the atmospheric pressure acting on the Earth’s surface is approximately 10135 N/m2, when the pressure inside a container is low, the atmospheric pressure acting on that container can reach one atmosphere. The force per unit area at different pressures is shown in Table 1. 　　(2) In a vacuum state, due to the low density of gas, the number of gas molecules per unit volume, that is, the molecular density of the gas, is less than that of gas under atmospheric pressure. Therefore, the number of collisions between molecules, between molecules and other particles (such as electrons, ions, etc.), and between molecules and various surfaces (such as vessel walls) decreases relatively, resulting in an increased molecular free path for the gas. Table 2 shows the relationship between the average free path of atmospheric molecules at room temperature and atmospheric pressure. Table 1: Force per unit area at different pressures
Pressure (Pa) Force (kg/cm²)
10⁵ 1.033285 × 10³ 6.79755 × 10⁻²
5 × 10⁴ 6.79755 × 10⁻¹ 1.35951 × 10⁻²
3 × 10⁴ 4.07853 × 10⁻¹⁵ 6.79755 × 10⁻³
1 × 10⁴ 1.35951 × 10⁻¹¹ 1.35951 × 10⁻³

Table 2: Relationship between the average free path of atmospheric molecules and atmospheric pressure at room temperature
Atmospheric Pressure (Pa) Average Free Path (cm)
10⁵ 6.5 × 10⁻⁶ 1 × 10⁻³
5 × 10⁴ 3.5 × 10² 1 × 10⁻⁶
10³ 5 × 10⁻⁴ 5 × 10⁻⁵
10² 5 × 10⁻³ 1 × 10⁻⁹
1 × 10⁻¹⁵ 1 × 10⁰ 1 × 10⁻⁴
(3) In a vacuum state, due to the reduced molecular density, the concentrations of gases such as oxygen and hydrogen that make up the atmosphere (as well as the water content) also decrease relatively. Table 3 shows the components of a standard atmosphere. Table 1 Composition of the standard atmosphere
Component | Molecular weight | Volume percentage | Weight percentage | Partial pressure (Torr)
---|---|---|---|---
N2 (Nitrogen) | 28.01 | 47.08 | 75.52 | 93.44
O2 (Oxygen) | 31.99 | 20.94 | 23.14 | 159.20
Ar (Argon) | 39.98 | 0.93 | 1.28 | 87.10
CO2 (Carbon dioxide) | 44.01 | 0.04 × 10⁻² | 4.8 × 10⁻² | 2.4 × 10⁻¹*
Ne (Neon) | 20.18 | 1.82 × 10⁻³ | 1.3 × 10⁻³ | 1.4 × 10⁻²
He (Helium) | 4.00 | 5.24 × 10⁻⁴ | 6.9 × 10⁻⁹ | 4.0 × 10⁻³
Kr (Krypton) | 83.80 | 1.14 × 10⁻⁴ | 3.3 × 10⁻⁴ | 8.7 × 10⁻⁴
Xe (Xenon) | 131.30 | 8.7 × 10⁻⁶ | 3.9 × 10⁻³ | 6.6 × 10⁻⁵
H2 (Hydrogen) | 2.0159 | 45 × 10⁻⁵ | 3.5 × 10⁻⁶ | 4 × 10⁻⁴
CH4 (Methane) | 16.04 | 30 × 10⁻⁴ | 1 × 10⁻⁴ | 1.5 × 10⁻³
N2O (Dinitrogen monoxide) | 44.01 | 85 × 10⁻⁵ | 8 × 10⁻⁴ | 4 × 10⁻³
O3 (Ozone) | 47.99 | 2 | Summer: 0–7 × 10⁻⁶; 0–1 × 10⁻⁵; 0–5 × 10⁻⁵* Winter: 0–2 × 10⁻⁶; 0–0.3 × 10⁻⁵; 0–1.5 × 10⁻⁵*
SO2 (Sulfur dioxide) | 64.06 | 280 | 0–1 × 10⁻⁴; 0–2 × 10⁻⁴; 0–8 × 10⁻⁴*
NO2 (Nitrogen dioxide) | 46.05 | 50 | 0–2 × 10⁻⁶; 0–3 × 10⁻⁶; 90–1.5 × 10⁻⁵*
NH3 (Ammonia) | 17.03 | 10 | Trace amounts; 0–Trace amounts; 0–Trace amounts
CO (Carbon monoxide) | 28.01 | 50 | Trace amounts; 0–Trace amounts; 0–Trace amounts
I2 (Iodine) | 253.80 | 88 | 0–1 × 10⁻⁶; 0–9 × 10⁻⁵; 0–8 × 10⁻⁶*

* Values that vary depending on time and location.

These characteristics of a vacuum have been utilized by people in numerous industrial and scientific experiments. This will be discussed in detail in the next section. 　　The meaning and characteristics of vacuum – Other related documents: The meaning and characteristics of vacuum; An overview of the application of various vacuum processing techniques in different vacuum conditions