Tungsten and Boron

The following stable intermediates exist W2B, WB, W2B5 j(, and V1 jfB3 (earlier WB4, WB12, W2 j B9). For more information on the tungsten-boron system and recent thermochemical data, see elsewhere [4.10]. 

PR By solid state reaction between W and amorphous B powder pressed in compacts at elevated temperature pretreatment at 500 °C in hydrogen for 1 horn followed by 800-1200 °C in argon for 2 hours. The formation of the individual boride depends on the W/B ratio. 

PP They all exhibit high hardness ( 9 Mohs), brittleness, and metal-like electrical conductivity. 

PP Occurs in a low-temperature (a) and high-temperature modification (P). Values for the transition temperature scatter in the literature are between 1950 and 2400 °C. 

---

Properties of the deposits Almost any material which can be melted is suitable for plasma spraying, giving a vast range of possible coatings of single or mixed metallic or non-metallic substances. It is often possible to produce types of coatings which are not obtainable in any other way. Typical of the materials which are plasma sprayed are copper, nickel, tantalum, molybdenum. Stellites, alumina, zirconia, tungsten and boron carbides, and stainless steels. 

Derivation Heating tungsten and boron in electric furnace. 

These data evidence that tungsten and boron hydrides are the most promising hydrogen accumulators of the hydrides studied the former being efficient in volume accumulation, the latter in weight one. The capacity of CeoHeo is close to that of boron tungsten. Both the hydrides may meet the demands of BMW car production to the mass capacity of hydrogen accumulators, namely it is to be not less than 10 %. 

Fig. 16. Insulator wall designs (a) peg wall (b) conducting bar wall and (c) segmented bar wall. The gas-side materials are tungsten and tungsten—copper composite, the base material, copper, and the insulators, boron nitride. Slagging grooves are shown.

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

The reduction of a transition-metal oxide and boron oxide by an electropositive metal such as Al, Mg or an alkali metal has been used as a pathway to titanium, iron, chromium, tungsten and alkali-earth borides . ... 

Marcantoncetos et al. [112] have described a phosphorimetric method for the determination of traces of boron in seawater. This method is based on the observation that in the glass formed by ethyl ether containing 8% of sulfuric acid at 77 K, boric acid gives luminescent complexes with dibenzoylmethane. A 0.5 ml sample is diluted with 10 ml 96% sulfuric acid, and to 0.05-0.3 ml of this solution 0.1ml 0.04 M dibenzoylmethane in 96% sulfuric acid is added. The solution is diluted to 0.4 ml with 96% sulfuric acid, heated at 70 °C for 1 h, cooled, ethyl ether added in small portions to give a total volume of 5 ml, and the emission measured at 77 K at 508 nm, with excitation at 402 nm. At the level of 22 ng boron per ml, hundredfold excesses of 33 ionic species give errors of less than 10%. However, tungsten and molybdenum both interfere. 

For the routine determination of analytes in the quality control of the production of speciality chemicals, a combination of direct current plasma emission spectroscopy (DCP-OES) with flow injection analysis (FIA) has been used. Results obtained for the determination of boron, copper, molybdenum, tungsten and zinc in non-aqueous solutions have been published by Brennan and Svehla [3], The principle has been extended to other analytes, carrier liquids, and solvents, and the details of a fully automatic system have been described by Brennan et al. [4]. 

CARBIDES. A binary solid compound of carbon and another element. The most familiar carbides are those of calcium, tungsten, silicon, boron, and iron (cemcntitc) Two factors have an important bearing on the properties of carbides (1) the difference in electronegativity between carbon and the second elemenl. and (2) whether the second element is a transition metal. Saltlike carbides of alkali metals are obtained by reaction with acetylene. Those ohlained from silver, copper, and mercury sails are explosive. See also Carbon and Iron Metals, Alloys, and Steels. 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

Tungsten trioxide, like molybdenum trioxide, possesses in marked degree the property of combining with other acidic oxides, such as phosphorus pentoxide, arsenic pentoxide, silica, and boron trioxide, in varying proportions, producing heteropolyacids which contain the VOL, VII. III. 193 13... 

Tungsten and Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth—Complex Salts containing Vanadium—Carbides—Complex Cyanogen Derivatives— Compounds with Silicon, Titanium, 2areonium, and Boron. 

---