Hydrogen is a colorless, odorless, and tasteless gas that is non-toxic and highly flammable. Its melting point and boiling point are -259.2℃ and -252.77℃, respectively, and its relative density is 0.06960 (at 0℃, air=1). The gas density of hydrogen is 0.08342 kg/ m³ (at 21.1℃ and 101.3kPa), while its liquid density is 70.96 kg/m³ (at -252.8℃ and 101.3kPa). At a critical temperature of 239.9 ℃ and a critical pressure of 1.297 MPa, hydrogen can dissolve in approximately 50 volumes of water at 0℃.
However, hydrogen can be asphyxiant at high concentrations. It is known to easily diffuse and penetrate, leading to its classification as a strong reducing agent that can penetrate steel and even cause hydrogen embrittlement. The hydrogen molecule consists of two isomers, with the ratio of positive to secondary hydrogen at 75:25 at room temperature. As the temperature drops, the proportion of secondary hydrogen increases, accompanied by the release of conversion heat. At a balance composition of 0.2:99.8, hydrogen is still non-toxic, but it cannot sustain life.
When mixed with air, hydrogen can form an explosive mixture that can lead to combustion and explosion in the presence of open fire and high heat. It may also react violently with fluorine, chlorine, and other dangerous elements. During combustion, there is no visible flame, and mixing hydrogen with air, oxygen, or chlorine makes it prone to explosion. The autoignition temperature of hydrogen is 571.2℃, while its flammability limit in air is at a range of 4.0%~75.0%.
The primary usage of high purity hydrogen lies in the production of large-scale integrated circuits wherein it plays a crucial role in creating a reducing atmosphere for the preparation of various gas mixtures such as SiH4/H2, PH3/H2, B2H6/H2, etc. Moreover, when it comes to manufacturing electric vacuum materials and devices, stringent purity standards for hydrogen are necessary. Special hydrogen burning treatment is carried out on metal components like the anode, cathode, and grid of electron tubes. Furthermore, hydrogen serves as a protective gas as well as a carrier gas during the epitaxial process of silicon and gallium arsenide.
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