Metallic tungsten

Wolframit, n. (Min.) wolframite. Wolfram-lampe, /. tungsten lamp, metall, n. metallic tungsten, -ocker, m. tungstic ocher, tungstite. -oxyd, n. tungsten oxide, -salz, n. tungsten salt. 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

Metallic taste, 11 565 Metallic tungsten, 25 374 Metallic Type II superconductors, critical current density value in, 23 822 Metallic vanadates, 25 513 Metalliding, 15 251 Metalliferous oxides deposits of, 17 689-690 in ocean basins, 17 693 Metalliferous sulfide deposits, 17 690-691 Metalliferous sulfides, in ocean basins, 17 693-694... 

Tungsten trioxide, in general, can be made by heating metallic tungsten, its carbides, its lower oxides, or tungstic acid in air. 

The publications and correspondence of Bergman and Scheele contain interesting allusions to the de Elhuyar brothers, to Hjelm, and to the early history of the metals tungsten and molybdenum which they discovered. The presence of a new metal in pitchblende was recognized by Klaproth in 1789, but it remained for Peligot half a century later to isolate uranium. Chromium, now the most familiar element of the group, was the last to be discovered when the immortal French chemist Vauquelin finally isolated it in 1798 from a Siberian mineral. For further information about tungsten see pp. 284-301. 

In 1783 the brothers collaborated in a research on tungsten and wolfram, and found that both these ores contained the tungstic acid that Scheele had reported. The first metallic tungsten was prepared not from... 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

Other Metals Tungsten, Molybdenum, Antimony, Brass, Tin, Berrylium, Precious Metals, Aluminum, Silicon, Nickel... 

The carbides and nitrides of the early transition metals have attracted considerable attention. Much of this can be attributed to the catalytic properties of these materials. Levy and Boudart1 were the first to note the Pt-like catalytic behavior of WC, which for many reactions is more catalytically active than metallic tungsten.2 Others have noted that the early transition metal carbides and nitrides, in general, are excellent catalysts for reactions characteristic of the more expensive noble metals.3,4 For example, WC has been found to catalyze the isomerization of neopentane to isopentane,1 a reaction that previously had been known to be catalyzed only by iridium and platinum.5... 

Compared to the corresponding carbides the heats of oxygen chemisorption on metals are higher. For example, on metallic tungsten the heat of adsorption is 812 kJ/mole 02, while on metallic chromium it is 730 kJ/ mole 02n. These values are significantly higher that those of the carbides of the same metals (Table 16.2). Thus, carbon atoms, when implanted in the metal lattice, reduce the adsorption affinity of the metal atoms towards oxygen. 

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