In addition to oxygen, nitrogen, and hydrogen, air contains trace amounts of rare gases such as neon, helium, krypton, and xenon. By volume fraction, neon accounts for approximately 15×10⁻⁶ to 18×10⁻⁶, helium for 4.6×10⁻⁶ to 5.3×10⁻⁶, krypton for only 1.08×10⁻⁶, and xenon for 0.08×10⁻⁶. Xenon is commonly known as the "golden gas." Due to their extremely low concentrations and the complexity of the extraction process, recovery units are only considered for oxygen plants with a capacity greater than 10,000 m³/h.
Neon and helium have very low liquefaction temperatures. At atmospheric pressure, the liquefaction temperature of neon is 27.26 K, and that of helium is 4.21 K. Neon is highly inert, making liquid neon a very safe coolant for cryogenic laboratories. At liquid helium temperatures, conductors lose their electrical resistance, allowing current to pass without loss, resulting in "superconductivity," which can be utilized to build superconducting motors. Therefore, with the advancement of ultra-low temperature technology, liquid helium will play an increasingly important role.
Helium is highly inert. It is used as a protective gas in the smelting of special rare metals such as titanium and zirconium, as well as semiconductors like silicon and germanium. Helium is also required as a shielding gas for welding and cutting high-grade alloys with high melting points and large thicknesses.
Helium has strong diffusivity and particularly high permeability. Therefore, for pressure vessels and vacuum systems with extremely stringent requirements, helium is the best leak detection indicator. In addition, helium is the optimal refrigerant for ultra-low temperature refrigerators. Helium liquefiers and helium refrigerators can achieve temperatures approaching absolute zero. Pumps operated with liquid helium can achieve the high vacuum of 133.32×10⁻⁹ Pa required in the electronics industry, and the ultra-high vacuum of 133.32×10⁻¹⁰ to 133.32×10⁻¹² Pa needed for space research.
In atomic physics, the helium nucleus is used as an alpha particle. In the atomic industry, helium is widely used as a protective gas. In nuclear reactors, helium serves not only as a protective gas but also as a coolant. Because helium is chemically inert and non-corrosive to combustion equipment, it can increase the reactor temperature and efficiency. Furthermore, helium itself has high thermal conductivity, providing excellent cooling performance.
In medicine, a 1:4 mixture of oxygen and helium can rapidly penetrate the lungs, accelerating the exchange of oxygen and carbon dioxide. It can be used to treat asthma, tracheal and laryngeal diseases, as well as decompression sickness.
In diving operations, if ordinary air is used, nitrogen dissolved in the blood can cause narcosis at depths below 50 m, posing a life-threatening risk to divers. Therefore, divers working at great depths cannot use pure oxygen; instead, an oxygen-helium mixture is used to replace air for breathing, ensuring safe operations at depths of up to 200 m. Consequently, helium consumption is substantial.
Since helium is safer than hydrogen, it can be used to replace hydrogen for filling airships and weather balloons. Helium can also be used as a carrier gas in chromatography.
With the development of space technology, laser technology, and infrared detection technology, helium has a wide range of applications.
Neon emits a red glow when filled in light bulbs and has long been used to fill neon signal devices and various discharge tubes. It is also widely used in laser technology and infrared detection.
The latent heat of vaporization of neon is 40 times greater than that of helium, making it suitable as a refrigerant for ultra-low temperatures, with a minimum temperature of -245.9°C. Neon and helium can also be used for measuring the true density and surface area of porous substances.
Krypton and xenon are mainly used in electric light sources. Bulbs filled with krypton, xenon, and argon mixtures are compact, long-lasting, and highly efficient-generally 4 to 5 times more efficient than incandescent lamps, with a lifespan increased by 2 to 3 times. Flash lamps and stroboscopes also utilize krypton and xenon. Because the discharge intensity of xenon lamps exceeds that of sunlight, long-arc xenon lamps filled with xenon, commonly known as "little suns," have extremely strong fog-penetrating capability. They can be used for lighting at airports, railway stations, docks, and other locations, as well as on battlefields.
Additionally, xenon has a relatively large molecular weight and strong anesthetic properties, making it an ideal anesthetic in medicine. Xenon also has the property of being impermeable to X-rays, and is used as a contrast agent for cerebral X-ray photography, as well as for X-ray shielding.
