Helium/Gas/Liquid Helium

The element helium, with the symbol He, is located in the second position of the periodic table and has an atomic weight of 4.002602. It is mainly composed of helium-4 isotopes. In nature, the content of helium is negligible, accounting for only 0.0005% of the air. As an inert rare gas, helium is colorless, odorless, tasteless, and has extremely low solubility in water, making it the least dense known gas except for hydrogen. [2]
The liquefaction difficulty of helium is extremely high, as it is the only substance that cannot solidify at standard atmospheric pressure. When liquid helium is cooled to below -270.98 ℃, it exhibits extremely low surface tension and strong thermal conductivity, with a thermal conductivity efficiency 800 times that of copper; In addition, its specific heat capacity, surface tension, and compressibility all exhibit anomalies and almost no viscosity. The application of liquid helium enables low temperatures close to absolute zero (-273.15 ℃) to be achieved, and its chemical properties are extremely stable, neither burning nor assisting combustion. [2]
The ultra-low temperature characteristics of liquid helium have revealed a series of fascinating physical phenomena, becoming the key to low-temperature physics experiments. Global physicists are dedicated to exploring material changes at lower temperatures through liquid helium to reveal unknown properties. This research field has given birth to a new branch of physics - low-temperature physics.