Tungsten elemental

The SIGMA telescope (Paul et al. 1991, Mandrou et al. 1992) is composed of a position sensitive Anger type gamma camera (GCA), sensitive to photons between 30 and 1300 keV, and a 2.5 cm thick coded mask of square tungsten elements of 9.4 mm size placed 2.5 m from the detector plane. The GCA is a single circular 1.25 cm thick Nal(Tl) crystal of 57 cm diameter, viewed... 

Mask Unit located at 540 mm from the detector plane and parallel to it, consisting of a static coded aperture mask made from 24x24 mm tungsten elements, 1 mm thick, within a carbon fibre honeycomb plate and support structure. 

The BCC structure is illustrated in figure Al.3,3. Elements such as sodium, tungsten and iron fonn in the BCC structure. The conventional unit cell of the BCC structure is cubic, like FCC, with the length of the edge given by the lattice parameter, a. There are two atoms in the conventional cell. In the primitive unit cell, there is only one atom and the lattice vectors are given by... 

Before it was known that elements beyond uranium were capable of existence, the heaviest known natural elements, thorium, protactinium and uranium, were placed in a sixth period of the periodic classification, corresponding to the elements hafnium, tantalum and tungsten in the preceding period. It was therefore implied that these elements were the beginning of a new, fourth transition series, with filling of the penultimate n = 6 level (just as the penultimate = 5... 

Tantalum is a gray, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below ISOoC, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. The element has a melting point exceeded only by tungsten and rhenium. Tantalum is used to make a variety... 

The element is silvery white with a metallic luster its density is exceeded only by that of platinum, iridium, and osmium, and its melting point is exceeded only by that of tungsten and... 

Chrome—nickel alloy heating elements that commonly ate used in low temperature furnaces are not suitable above the very low end of the range. Elements commonly used as resistors are either silicon carbide, carbon, or high temperature metals, eg, molybdenum and tungsten. The latter impose stringent limitations on the atmosphere that must be maintained around the heating elements to prevent rapid element failure (3), or the furnace should be designed to allow easy, periodic replacement. 

Density is a particularly important characteristic of alloys used in rotating machinery, because centrifugal stresses increase with density. Densities of the various metals in Table 1 range from 6.1 to 19.3 g/cm. Those of iron, nickel, and cobalt-base superaHoys fall in the range 7-8.5 g/cm. Those alloys which contain the heavier elements, ie, molybdenum, tantalum, or tungsten, have correspondingly high densities. 

Incandescent Lamps, Electronic Tubes, and Resistance Elements. Articles fashioned in any form from molybdenum and tungsten usually fall within the bounds of powder metallurgy. These metals normally are first produced as a powder. Both molybdenum and tungsten are used as targets in x-ray tubes, for stmctural shapes such as lead and grid wires in electron tubes, and as resistance elements in furnaces. 

Tungsten with the addition of as much as 5% thoria is used for thermionic emission cathode wires and as filaments for vibration-resistant incandescent lamps. Tungsten—rhenium alloys are employed as heating elements and thermocouples. Tantalum and niobium form continuous soHd solutions with tungsten. Iron and nickel are used as ahoy agents for specialized appHcations. 

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

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