Refractory metals thorium

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. 

Decontamination of equipment is identical to that described in the synthesis of calcium metal. A similar procedure can be used to produce powders of zirconium, chromium, vanadium, thorium, uranium, and other refractory metals from their oxides the reductant/metal oxide ratio, however, differs for each metal in order to avoid formation of oxygenated metal by-products and ensure maximum product purity. 

The hot-wire process was developed by Van Arkel and de Boer [V2], who used it to produce the first pure, massive specimens of many refractory metals, notably titanium, zirconium, hafnium, and thorium. An interesting account of early uses of this process is given in... 

Thoria has been used for crucibles for the melting of refractory metals but it has poor thermal-shock resistance. Thorium Sulphides. Three sulphides have been reported Th4S7, ThjSg and ThS. Crucibles made of these sulphides have been used as containers for molten Ce. 

In early 1965, the Australian chemist John Willis (17) showed that a burner using nitrous oxide and acetylene would develop enough energy to dissociate the compounds of many of the refractory elements. The temperature of the nitrous-oxide flame is thought to be about 2900°C. With it, routine analyses have been established for nearly all the remaining metallic elements the only ones which have so far resisted determination are cerium and thorium. 

The electrolysis cell is best constructed of a refractory of high alumina content, and containing as little silica as possible, owing to the ready attack of silica by thorium metal. The small-scale cell illustrated in Fig. 7.6, however, uses a silica pot and care is taken to prevent thorium metal contacting the silica wall or base. The small, 6 kg scale, model is heated externally, although a more conventional internal heating system is necessary for a production scale cell. 

Although some of these saltlike nitrides have high melting points (for instance, thorium nitride 2820"C uranium nitride 2800°C plutonium nitride 2550 beryllium nitride 2200 C barium nitride 2200"C), they are sensitive to hydrolysis and react readily with water or moisture to give ammonia and the corresponding metal oxide or hydroxide. Consequently, they do not meet the refractory requirements as interpreted here. Some of these nitrides are useful industrial materials particularly as sintering additives for the production of silicon nitride, aluminum nitride, and cubic boron nitride (see Ch. 14). 

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