Tungsten FLUORINE

The reactivity of fluorine compounds varies from extremely stable, eg, compounds such as sulfur hexafluoride [2551-62 ] nitrogen trifluoride [7783-54-2] and the perfluorocarbons (see Fluorine compounds, organic) to extremely reactive, eg, the halogen fluorides. Another unique property of nonionic metal fluorides is great volatiUty. Volatile compounds such as tungsten hexafluoride [7783-82-6] molybdenum hexafluoride [7783-77-9] ... 

Fluorination of tungsten and rhenium produces tungsten hexafluoride, WF, and rhenium hexafluoride [10049-17-9J, ReF, respectively. These volatile metal fluorides are used in the chemical vapor deposition industry to produce metal coatings and intricately shaped components (see Thin films,... 

The catalyst for the second stage is also a bifimctional catalyst containing hydrogenating and acidic components. Metals such as nickel, molybdenum, tungsten, or palladium are used in various combinations and dispersed on sofid acidic supports such as synthetic amorphous or crystalline sihca—alumina, eg, zeofites. These supports contain strongly acidic sites and sometimes are enhanced by the incorporation of a small amount of fluorine. 

Tungsten oxytetrafluofide [13520-79-17, WOF, mp 110°C, bp 187.5°C, forms colorless plates. It is prepared by the action of an oxygen—fluorine mixture on the metal at elevated temperatures (7). The compound is extremely hygroscopic and decomposes to tungstic acid in the presence of water. 

Tungsten disilicide [12039-88-2] WSi2, forms bluish-gray tetragonal crystals (a = 0.3212 nm, c = 0.7880 nm). It is insoluble in water and melts at 2160°C. The compound is attacked by fluorine, chlorine, fused alkaUes, and HNO —HF. It may be used for high temperature thermocouples in combination with MOSi in an oxidising atmosphere. 

The second group is the group of oxyfluorides that are derived from ferroelectric oxides by means of fluorine-oxygen substitution. The basic oxides are usually perovskite, tetragonal tungsten bronze, pyrochlore, lithium tantalate etc. [400]. 

C = carbon F = fluorine Si = silicon Cl = chlorine V = vanadium Fe = iron Cu = copper Br = bromine W = tungsten... 

Monocaesium acetylide and caesium acetylide, lithium acetylide and rubidimn acetylide, tungsten carbide and ditungsten carbide, and zirconium dicarbide all ignite in cold fluorine, while uranium dicarbide ignites in warm fluorine. 

Lead(II) oxide tungsten, zirconium Chlorinated rubber, chlorine, ethylene, fluorine, glycerol, metal acetylides, perchloric acid... 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

The gaseous product (SF6) does not interfere with the analysis of solids that form in the main bomb process. Furthermore, the contribution of reaction 7.82 to A f/jgp can be accurately calculated lfom the mass of sulfur and the value of ArC/°(7.82) at 298.15 K [124], It may also be necessary, in some cases, to use an auxiliary substance to promote complete combustion of the compound under study [111,113]. Tungsten is a very common combustion aid it reacts cleanly with excess fluorine according to... 

Tungsten has been used to induce complete combustions, for example, in the studies ofWTe2 [123],Mo5Si3 [125],andSi3N4 [126], In these cases, the sample was typically arranged as shown in figure 7.12. The compound under study (G), with the sulfur fuse sprinkled on top was placed in a thin tungsten saucer (I), supported by a nickel crucible (H) with holes in the sides (K) to allow rapid circulation of the fluorine around the combustible materials as the reaction proceeded. 

Tungsten films can be etched easily in fluorine-containing plasma 73,80). If carbon or sulfur is present in the gas atmosphere, oxygen is typically added to prevent polymer and residue formation and to increase the concentration of fluorine atoms. 

The mngsten ore called scheehte is named after Carl Wilhelm Scheele (1742—1786), who smdied and experimented with tungsten minerals, but as with many of his other near discoveries, such as oxygen, fluorine, hydrogen sulfide, hydrogen cyanide, and manganese, he was not given credit. 

Tungsten is a weak reducing agent ( ow/wo3 = —0.09 V), which can be oxidized into WOj, W2O5, or WO3. Passivation is achieved in aqueous solutions by WO3 which is stable at pH lower than 4 except in the presence of HF, which forms fluorinated tungsten complexes. Never-therless corrosion is very low in diluted ammonia. 

If a single metal film is deposited on an oxide, the sheet resistance measurement results can by easily interpreted and converted to the thickness. In practice, however, this is not usually the case. For example, in W CVD, the tungsten is not directly deposited on oxide due to high residual stress and unreliable adhesion. A titanium (Ti) layer must be first deposited as a glue layer. In addition, to prevent the fluorine in the CVD-precursor WFg from directly reacting with Ti (a strong catalytic reaction will occur), a barrier layer of titanium nitride (TiN) must be deposited on top of the Ti. As a result, we have a trilayer film of W on TiN on Ti on oxide, as shown schematically in Fig. 21. This poses some problems in accuracy in the four-point probe measurements. Based on the resistivities in Table VI, the... 

Thorium nitrate is a reagent for measuring fluorine and for making thori-ated tungsten filaments. Thorium nitrate containing 1% cerium nitrate is the impregnating liquid in making incandescent gas mantles. 

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