Tungsten refractory metals

The basic device is very simple. A tip of refractory metal, such as tungsten, is electrically heat-polished to yield a nearly hemispherical end of about 10" cm radius. A potential of about 10 kV is applied between the tip and a hemispherical fluorescent screen. The field, F, falls off with distance as kr, and if the two radii of curvature are a and b, the total potential difference V is then... 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

Both molybdenum and tungsten can be worked in air without ductiHty loss. AH refractory metals can be made into tubing by extmsion, and most refractory metals, except chromium, are available as wine. Tungsten wines were attempted as fiber reinforcement for experimental nickel-base composites. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Lubricants. TeUurides of titanium, 2irconium, molybdenum, tungsten, and other refractory metals are heat- and vacuum-stable. This property makes them useful in soUd self-lubricating composites in the electronics, instmmentation, and aerospace fields (see Lubrication and lubricants). Organic teUurides are antioxidants in lubricating oUs and greases. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Porous refractory (tungsten) infiltrated with a low melting point metal (silver) Hot-pressed refractory metal containing an oxide filler... 

The silicides of major industrial importance are the disilicides of the refractory metals molybdenum, tantalum, titanium, tungsten, vanadium, and zirconium.pl] These compounds are of great interest par-... 

The refractory metals for which CVD is commonly used to produce free-standing shapes are tungsten, niobium, rhenium, tantalum, molybdenum, and nickelb lb lb l (see Ch. 6). Shapes presently produced include rods, tubes, crucibles, manifolds, ordnance items, nozzles, and thrust chambers. They are usually deposited on a disposable mandrel of copper, molybdenum, or graphite which is subsequently machined off or removed chemically by etching. 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

Hafnium is used in control rods for nuclear reactors. It has high resistance to radiation and also very high corrosion resistance. Another major application is in alloys with other refractory metals, such as, tungsten, niobium and tantalum. 

In preparing solid platinum from its powder, Wollaston foreshadowed modern methods of powder metallurgy, by which the powders of refractory metals, such as tungsten, molybdenum, tantalum, and columbium, can be fabricated into useful articles (84, 86). 

Several other types of atomizer have been developed. Some of these are based on the design of the West rod, but others have made tubular atomizers from extremely refractory metals such as tungsten, tantalum and molybdenum. This latter class of atomizers tend to be made in-house by some laboratories and, at present, do not have any commercial suppliers. They have the advantage of being inert and non-porous so there is little interaction with the analyte, so that they can be used for the determination of elements which form refractory carbides. However, after extended use and in the presence of some acids, many of these atomizers become brittle and distorted. 

The best colour contrast of red blood cells is achieved on mono- and polycrystalline silicon, and monocrystalline surfaces of refractory metals molybdenum and tungsten, which possess maximal reflectivity. 

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