![]() ![]() Natural gas is a naturally occurring mixture of hydrocarbons and other compounds. These cryogenic applications depend crucially on the unique properties of helium and can be expected to ensure a demand for helium for the foreseeable future. Studies of materials at liquid helium temperatures or using liquid-helium-cooled apparatus play a central role in modern materials research. Another important application for liquid helium is the 3He- 4He dilution refrigerator, which employs both isotopes and allows routine access to temperatures within a few tens of millidegrees above absolute zero. A practical aspect of superfluid helium is its extremely high thermal conductivity, which is orders of magnitude greater than that of other excellent thermal conductors (e.g., copper). The superfluid properties of liquid 4He are generally believed to be a manifestation of the phenomenon known as the Bose-Einstein condensation, where the whole liquid exhibits macroscopic quantum properties such as quantization of the superfluid flow field. Liquid 4He undergoes a phase transition to a superfluid state when the temperature is lowered below 2.2 K. Liquid helium is also a subject of great scientific interest in itself. Another important use for liquid helium is in the cooling of superconducting magnets, an application of increasing importance with the advent of such technologies as magnetic resonance imaging (MRI) and superconducting cavities for high-energy accelerators (see Chapter 3). The fact that it remains a gas at liquid hydrogen temperatures makes it especially useful in the purging and pressurization of liquid hydrogen rocket propulsion systems. Helium's low liquefaction temperatures make it desirable for purging, pressurization, and cryogenic applications. An example in which these properties play a vital role is the manufacture of optical fibers, where the high thermal conductivity is important during the heat-treatment phase of fabrication, and the rapid diffusion of helium through the glass ensures that there are no trapped bubbles that would destroy the desired properties of the fibers. Second, helium atoms share with hydrogen the ability to diffuse with relative ease through many solid materials, especially at elevated temperatures. First, the thermal conductivity of gaseous helium is five to six times greater than that of other gases (the exception is hydrogen, which is comparable in thermal conductivity). There are, however, two other properties of helium that are important in some special industrial processes. ![]() The properties of helium that make it desirable for application as a lifting gas (i.e., its chemical inertness and low mass) also underlie its use for many other commercial applications. The inertness of helium firmly established it as the lifting gas of choice for most applications. Use, as was dramatically demonstrated when the zeppelin Hindenburg exploded and was then destroyed by fire at Lakehurst, New Jersey, in 1937. Although hydrogen provides about 7 percent more lift than helium, it is much more dangerous to The first large-scale use of helium for nonscientific purposes was its substitution for hydrogen as a lifting gas in lighter-than-air applications (e.g., balloons, zeppelins, and blimps). As will be discussed in Chapter 4, however, it is most economically obtained as a by-product of natural gas production. The heavier helium isotope, 4He, is the more abundant form and available from a number of sources, including Earth's atmosphere. This isotope is relatively rare and is obtained as a by-product of nuclear weapons production, following the radioactive decay of the hydrogen isotope tritium. Of the two stable helium isotopes, the lightest is 3He. At one atmosphere pressure, 4He liquefies at 4.2 K, whereas hydrogen liquefies at 20.4 K and neon at 27.1 K. The attractive forces between helium atoms are also so weak that helium has the lowest liquefaction temperature of all the "permanent" gases and, unlike all other elements, does not freeze under its own vapor pressure as the temperature is lowered toward absolute zero. As a result, helium is chemically inert and does not form stable compounds with other elements. The ionization potential for helium is higher than that for any other element. The two electrons of the helium atom form a closed spherical shell that is tightly bound to the nucleus. The nucleus of the atom consists of two protons and either one or two neutrons, depending on the isotope. It has a particularly stable and symmetrical structure. The helium atom is smaller than that of any other element and second only to the hydrogen atom in lightness. ![]()
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