hélium

Traductions

• anglais

  Date : 1983

  Définition

  An inert, gaseous substance, the lightest gas next to hydrogen. It is colourless, odourless and non-inflammable. Helium gas can be liquified only at very low temperature (4.2K) and helium liquid is the only known substance that cannot, at atmospheric pressure, be frozen solid - this can happen only at temperatures below 1°K at 25 atmospheres. It makes possible the study of temperatures near to absolute zeron and the phenomena of superconductivity and superfluidity. Liquid helium is the standard coolant for cryogenic devices.
  
  An element. State: gas group: noble gas; atomic no.: 2; atomic Wt.: 4.00260; valence: 0; isotopes: 2 stable. Helium (from Greek, meaning "the sun") was so named because of its original discovery in until 1895. It is component of natural gas, from which it is separately by compressing the gas at low temperatures until all the other components have been liquefied. Helium does not become liquid until a temperature of 4.2 Kelvin (about -265 °C) is reached and is thus a cryogenic liquid, which remains fluid at temperature near absolute zero.
  
  A colorless, odorless, inert element having a specific gravity of O.1368 and found in some natural gas.

  Notes :

  Helium has an extremely stable nucleus (2 protons, 2 neutrons) identical with the alpha particle emitted by radioactive substances. The radioactive decay of uranium and throrium in the earth produces helium which sometimes seeps into natural gas wells. The USA has sources of natural gas with a relatively high helium content but this amounts to no more than 7.5 per cent. Nevertheless, this is the principal source of helium. It is present in the atmosphere but only in negligible amounts. Even so, attemps have been made to extract it from air, possibly in a mixture with neon. Some designs of nuclear reactors use helium as coolant, the high-temperature gas-cooled reactor (HTGR) for example. Because helium absorbs virtually no neutrons it is a very attractive coolant for a fast breeder reactor, a gas cooled version of which is under development. In liquid metal-cooled fast breeder reactors, (LMFBR) helium is sometimes used as a 'blanket' on the open surfaces of the highly-reactive sodium or sodium-potassium alloy coolant.
  
  The nuclei of helium atoms are called alpha particles and are emitted during radioactive decay of unstable nuclei. Both gaseous and liquid forms can penetrate even dense solids three times as rapidly as air. Through helium cannot enter into chemical combination and thus forms no compounds, it has unusual properties asa liquid which have been the subject of much physical research. The chief uses of helium are as coolant for nuclear reactors, inflation of weather balloons, leak detection in high-vaccum equipment, arc-welding, as inert medium for semiconductor crystal growth, and as an aid in rocket and space vehicle launching. It is noncombustible and nontoxic.

  Termes :

  1. helium
  2. He