LYCOS RETRIEVER 
Helium (Element): Pressure
built 185 days ago
Webelements - Helium is uses for arc welding. Helium is cheaper than argon. Mixture of 80% helium and 20% oxygen is used for divers and others working under pressure. Helium is a semiconductor material. Helium is very rare in the earth. There is lot helium in the stars.
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Helium is a colorless, odorless, and non-toxic gas. It is the least reactive member of group 18 (the noble gases) of the periodic table and is inert and monatomic in virtually all conditions. Furthermore, the diameter of the helium atom is 0.98 Angstroms making it the world's smallest atom. Under standard temperature and pressure helium behaves very much like an ideal gas. Its abnormal thermal conductivity was disproved and then practically and theoretically clarified by the experiments of Pyotr Kapitsa (Nobel prize). [Eksperiment, teorija, praktika, Nauka Moscov 1977].
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Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium NMR spectroscopy. Many fullerenes containing helium-3 have been reported. These substances fit the definition of compounds in the Handbook of Chemistry and Physics.
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Helium nanodroplets are clusters composed typically of several thousand helium atoms held together by weak van der Waals forces. They can be formed by expanding helium gas at high pressure into vacuum through a pinhole nozzle cooled to ultra-low temperatures. They provide an extraordinary environment for studying molecules. The droplets are superfluid, and ... when a molecule is inserted into a helium nanodroplet, it is able to rotate and translate. Furthermore, the temperature within each droplet is exceedingly low, 0.4 K. These two properties combined make it possible to form and study unusual molecular complexes which are difficult, if not impossible, to study in any other environment.
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