Niobium combustion temperatures

In contrast, the combustion temperatures recorded for the tantalum-nitrogen and niobium-nitrogen systems were much lower than the melting points of the respective metals. Photographs of the cross-sections of undiluted samples do not indicate any macroscopic effect of melting, and neither do SEM photographs of the products. 

Figure 12.10 Niobium-nitrogen system Effect of nitrogen pressure and solid phase dilution on combustion temperature. Particle size 36 (rm.

For tungsten the analysis method is identical to that for niobium. The combustion temperature is 1350 to 1400°C. Burning is carried out in pure oxygen. Carbon dioxide is determined by conductometry after absorption in a 0.003 M sodium hydroxide solution. Any sulphur dioxide formed is adsorbed on manganese dioxide. Carbon monoxide is oxidized to carbon dioxide on copper oxide at 325°C. 

In a comprehensive study, Lassner (32) comes to the following conclusions the combustion of niobium, tantalum, molybdenum and tungsten in a stream of oxygen is easy, but to analyse carbon accurately the use of a flux is necessary. In the case of molybdenum it suppresses the sublimation of molybdenum trioxide, in the case of niobium and tantalum it lowers the reaction velocity. Larger pieces of these metals burn so quickly that the boats fuse and can thus not be used even with a flux. Tungsten, finally, burns slowly at a temperature below 1300°C, but so quickly above this temperature that for a short time very high temperatures are reached which cause melting on of the formed oxide. As it solidifies very quickly and encloses the rest of the metal, it causes low carbon results. 

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