Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2002) episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 480 HELIUM elongation 17 and Brinell hardness to 280. The coefficient of expansion is low 6 F 6 K . Heavy metal powder of Astro Alloys Corp. for making parts by powder metallurgy is prealloyed with the tungsten in a matrix of copper-nickel to prevent settling out of the heavy tungsten. HELIUM. A colorless odorless elementary gas He with a specific gravity of liquefying at -452 F C freezing at -458 F C . It has a valence of zero and forms no electron-bonded compounds. It has the highest ionization potential of any element. The lifting power of helium is only 92 that of hydrogen but it is preferred for ballons because it is inert and nonflammable and is used in weather balloons. It is also used instead of air to inflate large tires for aircraft to save weight. Because of its low density also it is used for diluting oxygen in the treatment of respiratory diseases. Its heat conductivity is about 6 times that of air and it is used as a shielding gas in welding and in vacuum tubes and electric lamps. Because of its inertness helium can be used to hold free chemical radicals which when released give high energy and thrust for missile propulsion. When an electric current is passed through helium it gives a pinkish-violet light and is thus used in advertising signs. Helium can be obtained from atmospheric nitrogen but comes chiefly from natural gas the gas of Texas yielding with some gases yielding as much as 2 . It also occurs in the mineral cleveite. Helium is transported as a liquid in trucks and tube trailers. Helium marketed by Matheson Gas Products Co. for use in semiconductor production and where noncontaminating atmospheres are needed is pure. In carbon dioxide lasers helium is used to cool the laser cavity. It is also used singly or in mixtures with argon or argon and carbon dioxide as a shielding gas in welding. HEMLOCK. The wood of the coniferous tree Tsuga mertensiana of the northeastern United States. This species
