Bronzes, sodium tungsten

Tungsten bronzes can be prepared by a variety of reductive techniques but probably the most general method consists of heating the normal tungstate with tungsten metal. They are extremely inert chemically, being resistant both to alkalis and to acids, even when hot and concentrated. Their colours depend in the proportion of M and W present. In the case of sodium... 

Conroy and Perlow [235] have measured the Debye-Waller factor for W in the sodium tungsten bronze Nao.gWOs. They derived a value of/= 0.18 0.01 which corresponds to a zero-point vibrational amplitude of R = 0.044 A. This amplitude is small as compared to that of beryllium atoms in metallic beryllium (0.098 A) or to that of carbon atoms in diamond (0.064 A). The authors conclude that atoms substituting tungsten in bronze may well be expected to have a high recoilless fraction. 

The gold paint with which lamp-posts and other ornamental metalwork are decorated contains no gold, because of its prohibitive expense. The reflective gold-coloured substitute is sodium tungsten bronze, Nao.3W03. So why is NaojWOs reflective ... 

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... 

Perform a similar experiment with sodium paratungstate, keeping the temperature within 600 °C. What properties do tungsten bronzes exhibit Where are they employed ... 

A clear crystallochemical interpretation of hardness anisotropy measurement results, especially for monocrystals, makes it possible to estimate the structural homogeneity of crystal. Button et al. (1979) testing the hardness of cubic sodium tungsten bronzes Na W03 (where 0.4 < x < 0.75) with the Knoop indenter, found the hardness of W03 to rise from 450 to 844 within a highly differentiated hardness anisotropy for various values of the Na+ ion. This variation is the outcome of differences in atomic spacings in crystals. 

Button T. W., Me Colm I. J., Wilson S. J., 1979, Hardness anisotropy and its dependence on composition in sodium tungsten bronzes and rhenium trioxide single crystals, J. Mater. Sci., 14, 159-164. 

It was recognized early (38) that the sodium-tungsten bronzes were vollkom-mene Leiter fur den electrischen Strom, though the first quantitative measurements of conductivity as a function of temperature indicated semiconducting behavior (14). Such semiconducting behavior continues to be reported (24)... 

The resistivities we find at room temperature are more than a factor of 10 lower than those reported by Ozerov for a single crystal at room temperature (26). Furthermore, our activation energies, 0.02 e.v., are considerably lower than the 0.12 e.v. derived from the data of Ozerov, although his thermal studies were all carried out on powders. Nevertheless, our conclusion is that the sodium-vanadium bronze is normally semiconducting and in this respect differs essentially from the equivalent sodium-tungsten bronzes. 

For the low ac-value sodium tungsten bronzes (x < 0.5), the crystal structure and x values of the crystals obtained depended strongly on the temperature of the melt. The size and homogeneity of the crystals were dependent on both temperature and electrode current. The best crystals were obtained at the lowest temperature at which they could be grown. The optimum current for best quality crystals depended upon x value and crystal structure. 

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